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 > levelOrderBottom(TreeNode *root) {
        vector > res;
        if(root == NULL) return res;
        
        queue q;
        q.push(root);
        while(!q.empty())
        {
            int levelSize = q.size();
            vector 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;
    }
};