Leetcode:層序遍歷，最優內存

/**
 * 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> > ans;
        if(root==NULL)return ans;
    	queue<TreeNode*> treeque;
    	treeque.push(root);
    	int number = 1;
    	TreeNode* cur;
    	int layer = 0;
    	
    	while(!treeque.empty()){
    		int i =  number;
    		number = 0;

    		ans.resize(layer+1);//初始化層數 

    		for(int j=0;j<i;j++){
    			cur = treeque.front();
    			treeque.pop();
    			ans[layer].push_back(cur->val);
    			if(cur->left!=NULL){
    				treeque.push(cur->left);
    				number++;
				}
    			if(cur->right!=NULL){
    				treeque.push(cur->right);
    				number++;
				}
			}//先讀這一層，再入下一層 
			layer++;
		}
		return ans;
    }
};