Binary Tree Level Order Traversal II

对于树的层次遍历，用STL queue来实现，我以前的方法一直是用一个level来判断是否到了新的一层；而这种方法直接能获知当前层的节点数，很好的方法，学习了。

这种方法也适用于任意的BFS.

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<vector<int> > levelOrderBottom(TreeNode *root) {        vector<vector<int> > res;        if(root == NULL) return res;                queue<TreeNode*> q;        q.push(root);        while(!q.empty())        {            int levelSize = q.size();            vector<int> row;            for(int i = 0; i < levelSize; i ++){                TreeNode *cur = q.front(); q.pop();                row.push_back(cur->val);                if(cur->left != NULL) q.push(cur->left);                if(cur->right != NULL) q.push(cur->right);            }            res.push_back(row);        }        reverse(res.begin(), res.end());                return res;    }};