Leetcode:Binary Tree Level Order Traversal

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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

 

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]   
分析：用两个vector分别保存本层和下一层的节点。代码如下：

class Solution {
public:
    vector<vector<int> > levelOrder(TreeNode *root) {
        vector<vector<int> > result;
        if(root == NULL) return result;
        
        vector<TreeNode *> prev;
        prev.push_back(root);
        
        while(!prev.empty()){
            vector<TreeNode *> cur;
            vector<int> level;
            
            for(auto i = prev.begin(); i != prev.end(); i++){
                level.push_back((*i)->val);
                if((*i)->left) cur.push_back((*i)->left);
                if((*i)->right) cur.push_back((*i)->right);
            }
            result.push_back(level);
            prev = cur;
        }
        
        return result;
    }
};