利用队列的先进先出的特点把每一层的元素push进去后在一次输出出来。
主要代码如下

/**
 * 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*> q;
        q.push(root);
         while(!q.empty())
         {
             int sz=q.size();
            vector<int>a;
            for(int i=0;i<sz;i++)
            {
                TreeNode* fro=q.front();
                q.pop();
                a.push_back(fro->val);
                if(fro->left)
                    q.push(fro->left);
                if(fro->right)
                    q.push(fro->right);
            }
            ans.push_back(a);
         }
        return ans;
    }
};