{"id":69,"date":"2017-09-04T00:53:41","date_gmt":"2017-09-04T07:53:41","guid":{"rendered":"http:\/\/zxi.mytechroad.com\/blog\/?p=69"},"modified":"2018-04-19T08:40:15","modified_gmt":"2018-04-19T15:40:15","slug":"leetcode-102-binary-tree-level-order-traversal","status":"publish","type":"post","link":"https:\/\/zxi.mytechroad.com\/blog\/leetcode\/leetcode-102-binary-tree-level-order-traversal\/","title":{"rendered":"\u82b1\u82b1\u9171 LeetCode 102. Binary Tree Level Order Traversal"},"content":{"rendered":"<p><iframe loading=\"lazy\" title=\"LeetCode 102. Binary Tree Level Order Traversal - \u82b1\u82b1\u9171 \u5237\u9898\u627e\u5de5\u4f5c EP33\" width=\"500\" height=\"375\" src=\"https:\/\/www.youtube.com\/embed\/Tuij96VBdu8?feature=oembed\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" referrerpolicy=\"strict-origin-when-cross-origin\" allowfullscreen><\/iframe><\/p>\n<p>Given a binary tree, return the\u00a0<i>level order<\/i>\u00a0traversal of its nodes&#8217; values. (ie, from left to right, level by level).<\/p>\n<p>For example:<br \/>\nGiven binary tree\u00a0<code>[3,9,20,null,null,15,7]<\/code>,<\/p>\n<pre class=\"\">    3\r\n   \/ \\\r\n  9  20\r\n    \/  \\\r\n   15   7\r\n<\/pre>\n<p>return its level order traversal as:<\/p>\n<pre class=\"\">[\r\n  [3],\r\n  [9,20],\r\n  [15,7]\r\n]<\/pre>\n<p>Solution 1: BFS O(n)<\/p>\n<pre class=\"lang:c++ decode:true \">class Solution {\r\npublic:\r\n    vector&lt;vector&lt;int&gt;&gt; levelOrder(TreeNode* root) {\r\n        if(!root) return {};\r\n        vector&lt;vector&lt;int&gt;&gt; ans;\r\n        vector&lt;TreeNode*&gt; curr,next;\r\n        curr.push_back(root);\r\n        while(!curr.empty()) {\r\n            ans.push_back({});\r\n            for(TreeNode* node : curr) {\r\n                ans.back().push_back(node-&gt;val);\r\n                if(node-&gt;left) next.push_back(node-&gt;left);\r\n                if(node-&gt;right) next.push_back(node-&gt;right);\r\n            }\r\n            curr.swap(next);\r\n            next.clear();\r\n        }\r\n        return ans;\r\n    }\r\n};<\/pre>\n<p>Solution 2: DFS O(n)<\/p>\n<pre class=\"lang:default decode:true \">class Solution {\r\npublic:\r\n    vector&lt;vector&lt;int&gt;&gt; levelOrder(TreeNode* root) {\r\n        vector&lt;vector&lt;int&gt;&gt; ans;\r\n        DFS(root, 0 \/* depth *\/, ans);\r\n        return ans;\r\n    }\r\nprivate:\r\n    void DFS(TreeNode* root, int depth, vector&lt;vector&lt;int&gt;&gt;&amp; ans) {\r\n        if(!root) return;\r\n        \/\/ Works with pre\/in\/post order\r\n        while(ans.size()&lt;=depth) ans.push_back({});\r\n        ans[depth].push_back(root-&gt;val); \/\/ pre-order\r\n        DFS(root-&gt;left, depth+1, ans);        \r\n        DFS(root-&gt;right, depth+1, ans);   \r\n    }\r\n};<\/pre>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Given a binary tree, return the\u00a0level order\u00a0traversal of its nodes&#8217; values. (ie, from left to right, level by level). For example: Given binary tree\u00a0[3,9,20,null,null,15,7], 3&#8230;<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3,45],"tags":[34,30,33,32],"class_list":["post-69","post","type-post","status-publish","format-standard","hentry","category-leetcode","category-tree","tag-bfs","tag-binary-tree","tag-dfs","tag-traversal","entry","simple"],"_links":{"self":[{"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/posts\/69","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/comments?post=69"}],"version-history":[{"count":4,"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/posts\/69\/revisions"}],"predecessor-version":[{"id":2696,"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/posts\/69\/revisions\/2696"}],"wp:attachment":[{"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/media?parent=69"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/categories?post=69"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/zxi.mytechroad.com\/blog\/wp-json\/wp\/v2\/tags?post=69"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}