学步园

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]
]

/**
 * 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> > levelOrder(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        
    	vector<int> level;
		vector<vector<int> > ans;
		if ( !root )
			return ans;

		queue<TreeNode* > Q;
		Q.push(root);
		int lsum=1;
		
		while(lsum>0)
		{
			int next=0;
			level.clear();
			while(lsum--)
			{
				TreeNode* tmp=Q.front();
				Q.pop();
				level.push_back(tmp->val);
				if ( tmp->left )
				{
					Q.push(tmp->left);
					next++;
				}
				if ( tmp->right)
				{
					Q.push(tmp->right);
					next++;
				}
			}
			ans.push_back(level);
			lsum=next;
		}
		return ans;
    }
};