1. 题目
Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).
For example:
Given binary tree [3,9,20,null,null,15,7],3
/ \
9 20/ \
15 7
return its level order traversal as:[ [3], [9,20], [15,7]]2. 思路
用FIFO队列来构建水平遍历。用NULL来分割不同的层。
3. 代码
/** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public: vector > levelOrder(TreeNode* root) { vector > ret; if (root == NULL) { return ret; } vector level; deque d; d.push_back(root); d.push_back(NULL); while (!d.empty()) { TreeNode* n = d.front(); d.pop_front(); if (n == NULL) { if (level.size() != 0) { ret.push_back(level); level.clear(); } if (d.empty()) { return ret; } else { d.push_back(NULL); } } else { if (n->left != NULL) { d.push_back(n->left); } if (n->right != NULL) { d.push_back(n->right); } level.push_back(n->val); } } return ret; }};