LeetCode-M-Binary Tree Level Order Traversal

题意

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]]

解法

利用队列，层次遍历

实现

/** * 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<vector<int>> levelOrder(TreeNode* root) {        vector<vector<int>> result;        if(root == NULL) return result;        queue<TreeNode*> tq;        tq.push(root);        tq.push(NULL);        while(!tq.empty() && tq.front() != NULL){            vector<int> sr;            while(tq.front() != NULL){                TreeNode* node = tq.front();                tq.pop();                sr.push_back(node->val);                if(node->left != NULL) tq.push(node->left);                if(node->right != NULL) tq.push(node->right);            }            tq.push(NULL);            result.push_back(sr);            tq.pop();        }        return result;    }};