二叉树的层序遍历

用队列实现，每次记录下当前队列的大小即可：

//
// Created by jt on 2020/8/22.
//
#include <vector>
#include <queue>
using namespace std;

class Solution {
public:
    /**
     *
     * @param root TreeNode类
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > levelOrder(TreeNode* root) {
        // write code here
        vector<vector<int> > res;
        if (!root) return res;
        queue<TreeNode *> qu;
        qu.push(root);
        while (!qu.empty()) {
            int size = qu.size();
            vector<int> vec;
            while (size--) {
                TreeNode *node = qu.front();
                qu.pop();
                vec.push_back(node->val);
                if (node->left) qu.push(node->left);
                if (node->right) qu.push(node->right);
            }
            if (vec.size() > 0) res.push_back(vec);
        }
        return res;
    }
};