{"id":449,"date":"2025-10-23T17:50:16","date_gmt":"2025-10-23T09:50:16","guid":{"rendered":"https:\/\/heyingnian.com\/?p=449"},"modified":"2025-10-23T17:50:54","modified_gmt":"2025-10-23T09:50:54","slug":"leetcode-105","status":"publish","type":"post","link":"https:\/\/heyingnian.com\/index.php\/2025\/10\/23\/leetcode-105\/","title":{"rendered":"\u4ece\u524d\u5e8f\u4e0e\u4e2d\u5e8f\u904d\u5386\u5e8f\u5217\u6784\u9020\u4e8c\u53c9\u6811"},"content":{"rendered":"\n<h1 class=\"wp-block-heading\">\ud83d\udd17 \u4ece\u524d\u5e8f\u4e0e\u4e2d\u5e8f\u904d\u5386\u5e8f\u5217\u6784\u9020\u4e8c\u53c9\u6811<\/h1>\n\n\n\n<p><strong>\u9898\u76ee\u6765\u6e90<\/strong>\uff1a<a target=\"_blank\" rel=\"noreferrer noopener\"><a href=\"https:\/\/leetcode.cn\/problems\/construct-binary-tree-from-preorder-and-inorder-traversal\/description\/?envType=study-plan-v2&amp;envId=top-100-liked\" target=\"_blank\" rel=\"noopener\">105. \u4ece\u524d\u5e8f\u4e0e\u4e2d\u5e8f\u904d\u5386\u5e8f\u5217\u6784\u9020\u4e8c\u53c9\u6811 &#8211; \u529b\u6263\uff08LeetCode\uff09<\/a><\/a><\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\ud83d\udccc \u9898\u76ee\u63cf\u8ff0<\/h2>\n\n\n\n<p>\u7ed9\u5b9a\u4e24\u4e2a\u6574\u6570\u6570\u7ec4 <code>preorder<\/code> \u548c <code>inorder<\/code>\uff0c\u5176\u4e2d\uff1a<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>preorder<\/code>\u00a0\u662f\u4e8c\u53c9\u6811\u7684<strong>\u5148\u5e8f\u904d\u5386<\/strong>\u5e8f\u5217\uff1b<\/li>\n\n\n\n<li><code>inorder<\/code>\u00a0\u662f\u540c\u4e00\u68f5\u6811\u7684<strong>\u4e2d\u5e8f\u904d\u5386<\/strong>\u5e8f\u5217\u3002<\/li>\n<\/ul>\n\n\n\n<p>\u8bf7<strong>\u6784\u9020\u8be5\u4e8c\u53c9\u6811\u5e76\u8fd4\u56de\u5176\u6839\u8282\u70b9<\/strong>\u3002<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p>\u5047\u8bbe\u6811\u4e2d<strong>\u65e0\u91cd\u590d\u5143\u7d20<\/strong>\uff0c\u4e14\u8f93\u5165\u5e8f\u5217\u4e00\u5b9a\u5408\u6cd5\u3002<\/p>\n<\/blockquote>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u2705 \u793a\u4f8b\u89e3\u6790<\/h2>\n\n\n\n<p><strong>\u793a\u4f8b 1\uff1a<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-preformatted\"><code>\u8f93\u5165: preorder = [3,9,20,15,7], inorder = [9,3,15,20,7]<br>\u8f93\u51fa: [3,9,20,null,null,15,7]<\/code><\/pre>\n\n\n\n<p>\u89e3\u91ca\uff1a<\/p>\n\n\n\n<figure class=\"wp-block-image size-full\"><div class='fancybox-wrapper lazyload-container-unload' data-fancybox='post-images' href='https:\/\/heyingnian.com\/wp-content\/uploads\/2025\/10\/image-11.png'><img class=\"lazyload lazyload-style-1\" src=\"data:image\/svg+xml;base64,PCEtLUFyZ29uTG9hZGluZy0tPgo8c3ZnIHdpZHRoPSIxIiBoZWlnaHQ9IjEiIHhtbG5zPSJodHRwOi8vd3d3LnczLm9yZy8yMDAwL3N2ZyIgc3Ryb2tlPSIjZmZmZmZmMDAiPjxnPjwvZz4KPC9zdmc+\"  loading=\"lazy\" decoding=\"async\" width=\"277\" height=\"302\" data-original=\"https:\/\/heyingnian.com\/wp-content\/uploads\/2025\/10\/image-11.png\" src=\"data:image\/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB\/AAffA0nNPuCLAAAAAElFTkSuQmCC\" alt=\"\" class=\"wp-image-450\"  sizes=\"auto, (max-width: 277px) 100vw, 277px\" \/><\/div><\/figure>\n\n\n\n<ul class=\"wp-block-list\">\n<li>\u5148\u5e8f\uff1a<code>[3, 9, 20, 15, 7]<\/code>\u00a0\u2192 \u6839 \u2192 \u5de6 \u2192 \u53f3<\/li>\n\n\n\n<li>\u4e2d\u5e8f\uff1a<code>[9, 3, 15, 20, 7]<\/code>\u00a0\u2192 \u5de6 \u2192 \u6839 \u2192 \u53f3<\/li>\n\n\n\n<li>\u6839\u4e3a\u00a0<code>3<\/code>\uff0c\u5de6\u5b50\u6811\u4e2d\u5e8f\u4e3a\u00a0<code>[9]<\/code>\uff0c\u53f3\u5b50\u6811\u4e2d\u5e8f\u4e3a\u00a0<code>[15,20,7]<\/code>\uff0c\u9012\u5f52\u6784\u5efa\u5373\u53ef\u3002<\/li>\n<\/ul>\n\n\n\n<p><strong>\u793a\u4f8b 2\uff1a<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-preformatted\"><code>\u8f93\u5165: preorder = [-1], inorder = [-1]<br>\u8f93\u51fa: [-1]<\/code><\/pre>\n\n\n\n<p><strong>\u793a\u4f8b 3\uff1a<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-preformatted\"><code>\u8f93\u5165: preorder = [1,2], inorder = [2,1]<br>\u8f93\u51fa: [1,2]<\/code><\/pre>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u26a0\ufe0f \u63d0\u793a<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>1 &lt;= preorder.length &lt;= 3000<\/code><\/li>\n\n\n\n<li><code>inorder.length == preorder.length<\/code><\/li>\n\n\n\n<li><code>-3000 &lt;= preorder[i], inorder[i] &lt;= 3000<\/code><\/li>\n\n\n\n<li>\u6240\u6709\u5143\u7d20<strong>\u552f\u4e00<\/strong>\uff0c\u4e14\u00a0<code>inorder<\/code>\u00a0\u662f\u00a0<code>preorder<\/code>\u00a0\u5bf9\u5e94\u6811\u7684\u4e2d\u5e8f\u904d\u5386<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\ud83d\udca1 \u89e3\u6cd5\uff1a\u9012\u5f52 + \u54c8\u5e0c\u8868\u4f18\u5316 \u2014\u2014 O(n) \u65f6\u95f4\uff0cO(n) \u7a7a\u95f4<\/h2>\n\n\n\n<pre class=\"wp-block-code\"><code>class Solution {\n    int&#91;] preorder; \/\/ \u5168\u5c40\u4fdd\u5b58\u5148\u5e8f\u6570\u7ec4\uff0c\u907f\u514d\u53c2\u6570\u4f20\u9012\n\n    public TreeNode buildTree(int&#91;] preorder, int&#91;] inorder) {\n        this.preorder = preorder;\n        Map&lt;Integer, Integer> map = new HashMap&lt;>();\n        \/\/ \u6784\u5efa\u4e2d\u5e8f\u503c\u5230\u7d22\u5f15\u7684\u6620\u5c04\uff0c\u52a0\u901f\u67e5\u627e\u6839\u4f4d\u7f6e\n        for (int i = 0; i &lt; inorder.length; i++) {\n            map.put(inorder&#91;i], i);\n        }\n        return dfs(0, 0, inorder.length - 1, map);\n    }\n\n    \/**\n     * @param preRoot  \u5f53\u524d\u5b50\u6811\u5728 preorder \u4e2d\u7684\u6839\u8282\u70b9\u7d22\u5f15\n     * @param inLeft   \u5f53\u524d\u5b50\u6811\u5728 inorder \u4e2d\u7684\u5de6\u8fb9\u754c\n     * @param inRight  \u5f53\u524d\u5b50\u6811\u5728 inorder \u4e2d\u7684\u53f3\u8fb9\u754c\n     * @param map      \u4e2d\u5e8f\u503c\u5230\u7d22\u5f15\u7684\u54c8\u5e0c\u6620\u5c04\n     * @return         \u6784\u5efa\u7684\u5b50\u6811\u6839\u8282\u70b9\n     *\/\n    public TreeNode dfs(int preRoot, int inLeft, int inRight, Map&lt;Integer, Integer> map) {\n        if (inLeft > inRight) {\n            return null;\n        }\n\n        int rootVal = preorder&#91;preRoot];\n        int rootIdx = map.get(rootVal); \/\/ \u6839\u5728\u4e2d\u5e8f\u4e2d\u7684\u4f4d\u7f6e\n\n        TreeNode node = new TreeNode(rootVal);\n\n        \/\/ \u5de6\u5b50\u6811\uff1a\n        \/\/ - \u5148\u5e8f\u6839\u7d22\u5f15\uff1apreRoot + 1\n        \/\/ - \u4e2d\u5e8f\u8303\u56f4\uff1a&#91;inLeft, rootIdx - 1]\n        node.left = dfs(preRoot + 1, inLeft, rootIdx - 1, map);\n\n        \/\/ \u53f3\u5b50\u6811\uff1a\n        \/\/ - \u5148\u5e8f\u6839\u7d22\u5f15 = preRoot + 1 + \u5de6\u5b50\u6811\u8282\u70b9\u6570\n        \/\/ - \u5de6\u5b50\u6811\u8282\u70b9\u6570 = (rootIdx - 1) - inLeft + 1 = rootIdx - inLeft\n        \/\/ - \u6240\u4ee5\u53f3\u5b50\u6811\u5148\u5e8f\u6839\u7d22\u5f15 = preRoot + 1 + (rootIdx - inLeft)\n        node.right = dfs(preRoot + 1 + (rootIdx - inLeft), rootIdx + 1, inRight, map);\n\n        return node;\n    }\n}<\/code><\/pre>\n\n\n\n<p>\u2705 <strong>\u5173\u952e\u70b9<\/strong>\uff1a<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>\u5229\u7528<strong>\u5148\u5e8f\u786e\u5b9a\u6839<\/strong>\uff0c<strong>\u4e2d\u5e8f\u5212\u5206\u5de6\u53f3\u5b50\u6811<\/strong>\uff1b<\/li>\n\n\n\n<li>\u7528\u54c8\u5e0c\u8868\u00a0<strong>O(1) \u5b9a\u4f4d\u6839\u5728\u4e2d\u5e8f\u4e2d\u7684\u4f4d\u7f6e<\/strong>\uff1b<\/li>\n\n\n\n<li>\u901a\u8fc7<strong>\u5de6\u5b50\u6811\u957f\u5ea6<\/strong>\u8ba1\u7b97\u53f3\u5b50\u6811\u5728\u5148\u5e8f\u4e2d\u7684\u8d77\u59cb\u4f4d\u7f6e\u3002<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\ud83d\udcca \u590d\u6742\u5ea6\u5206\u6790<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>\u65f6\u95f4\u590d\u6742\u5ea6<\/strong>\uff1a<code>O(n)<\/code>\n<ul class=\"wp-block-list\">\n<li>\u5efa\u54c8\u5e0c\u8868\uff1a<code>O(n)<\/code><\/li>\n\n\n\n<li>\u6bcf\u4e2a\u8282\u70b9\u88ab\u6784\u5efa\u4e00\u6b21\uff0c\u6bcf\u6b21\u9012\u5f52\u64cd\u4f5c\u00a0<code>O(1)<\/code>\uff0c\u5171\u00a0<code>n<\/code>\u00a0\u4e2a\u8282\u70b9 \u2192\u00a0<code>O(n)<\/code><\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>\u7a7a\u95f4\u590d\u6742\u5ea6<\/strong>\uff1a<code>O(n)<\/code>\n<ul class=\"wp-block-list\">\n<li>\u54c8\u5e0c\u8868\u5b58\u50a8\u00a0<code>n<\/code>\u00a0\u4e2a\u5143\u7d20 \u2192\u00a0<code>O(n)<\/code><\/li>\n\n\n\n<li>\u9012\u5f52\u6808\u6df1\u5ea6\u6700\u574f\u4e3a\u00a0<code>O(n)<\/code>\uff08\u6811\u9000\u5316\u4e3a\u94fe\u8868\uff09<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\ud83e\udde9 \u89e3\u9898\u601d\u8def<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">\u6838\u5fc3\u601d\u60f3\uff1a\u5206\u6cbb\u9012\u5f52<\/h3>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>\u5148\u5e8f\u904d\u5386\u7684\u7b2c\u4e00\u4e2a\u5143\u7d20\u4e00\u5b9a\u662f\u5f53\u524d\u6811\u7684\u6839\u8282\u70b9<\/strong>\u3002<\/li>\n\n\n\n<li>\u5728<strong>\u4e2d\u5e8f\u904d\u5386\u4e2d\u627e\u5230\u8be5\u6839\u7684\u4f4d\u7f6e<\/strong>\uff0c\u5176\u5de6\u4fa7\u4e3a\u5de6\u5b50\u6811\uff0c\u53f3\u4fa7\u4e3a\u53f3\u5b50\u6811\u3002<\/li>\n\n\n\n<li><strong>\u9012\u5f52\u6784\u5efa\u5de6\u53f3\u5b50\u6811<\/strong>\u3002<\/li>\n<\/ol>\n\n\n\n<h3 class=\"wp-block-heading\">\u5982\u4f55\u786e\u5b9a\u53f3\u5b50\u6811\u5728\u5148\u5e8f\u4e2d\u7684\u8d77\u59cb\u4f4d\u7f6e\uff1f<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>\u5148\u5e8f\u7ed3\u6784\uff1a<code>[\u6839][\u5de6\u5b50\u6811][\u53f3\u5b50\u6811]<\/code><\/li>\n\n\n\n<li>\u5de6\u5b50\u6811\u8282\u70b9\u6570 =\u00a0<code>rootIdx - inLeft<\/code><\/li>\n\n\n\n<li>\u6240\u4ee5\u53f3\u5b50\u6811\u5728\u5148\u5e8f\u4e2d\u7684\u8d77\u59cb\u7d22\u5f15 =\u00a0<code>preRoot + 1 + (rootIdx - inLeft)<\/code><\/li>\n<\/ul>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p>\ud83c\udf30 \u4e3e\u4f8b\uff1a<br><code>preorder = [3,9,20,15,7]<\/code>\uff0c\u5f53\u524d\u6839 <code>3<\/code> \u5728 <code>preorder[0]<\/code><br>\u4e2d\u5e8f\u4e2d <code>3<\/code> \u7684\u7d22\u5f15\u4e3a <code>1<\/code>\uff0c\u5de6\u5b50\u6811\u957f\u5ea6 = <code>1 - 0 = 1<\/code><br>\u2192 \u53f3\u5b50\u6811\u5728\u5148\u5e8f\u4e2d\u4ece <code>0 + 1 + 1 = 2<\/code> \u5f00\u59cb\uff0c\u5373 <code>20<\/code><\/p>\n<\/blockquote>\n\n\n\n<h3 class=\"wp-block-heading\">\u4e3a\u4ec0\u4e48\u9700\u8981\u54c8\u5e0c\u8868\uff1f<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>\u82e5\u6bcf\u6b21\u5728\u00a0<code>inorder<\/code>\u00a0\u4e2d\u7ebf\u6027\u67e5\u627e\u6839\u7684\u4f4d\u7f6e\uff0c\u65f6\u95f4\u590d\u6742\u5ea6\u4f1a\u9000\u5316\u4e3a\u00a0<code>O(n\u00b2)<\/code>\u3002<\/li>\n\n\n\n<li>\u54c8\u5e0c\u8868\u5c06\u67e5\u627e\u4f18\u5316\u4e3a\u00a0<code>O(1)<\/code>\uff0c\u6574\u4f53\u8fbe\u5230\u00a0<code>O(n)<\/code>\u3002<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>\ud83d\udd17 \u4ece\u524d\u5e8f\u4e0e\u4e2d\u5e8f\u904d\u5386\u5e8f\u5217\u6784\u9020\u4e8c\u53c9\u6811 \u9898\u76ee\u6765\u6e90\uff1a105. \u4ece\u524d\u5e8f\u4e0e\u4e2d\u5e8f\u904d\u5386\u5e8f\u5217\u6784\u9020\u4e8c\u53c9\u6811 &#8211; \u529b\u6263\uff08 [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[39,12],"tags":[57,58,14],"class_list":["post-449","post","type-post","status-publish","format-standard","hentry","category-binary_tree","category-algorithm","tag-binary_tree","tag-dfs","tag-algorithm"],"_links":{"self":[{"href":"https:\/\/heyingnian.com\/index.php\/wp-json\/wp\/v2\/posts\/449","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/heyingnian.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/heyingnian.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/heyingnian.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/heyingnian.com\/index.php\/wp-json\/wp\/v2\/comments?post=449"}],"version-history":[{"count":1,"href":"https:\/\/heyingnian.com\/index.php\/wp-json\/wp\/v2\/posts\/449\/revisions"}],"predecessor-version":[{"id":451,"href":"https:\/\/heyingnian.com\/index.php\/wp-json\/wp\/v2\/posts\/449\/revisions\/451"}],"wp:attachment":[{"href":"https:\/\/heyingnian.com\/index.php\/wp-json\/wp\/v2\/media?parent=449"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/heyingnian.com\/index.php\/wp-json\/wp\/v2\/categories?post=449"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/heyingnian.com\/index.php\/wp-json\/wp\/v2\/tags?post=449"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}