102. Binary Tree Level Order Traversal**

102. Binary Tree Level Order Traversal**

https://leetcode.com/problems/binary-tree-level-order-traversal/

题目描述

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,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

C++ 实现 1

层序遍历使用队列.

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        if (!root) return {};
        vector<vector<int>> res;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()) {
            vector<int> cur;
            int size = q.size();
            for (int i = 0; i < size; ++i) {
                auto node = q.front();
                q.pop();
                cur.push_back(node->val);
                if (node->left) q.push(node->left);
                if (node->right) q.push(node->right);
            }
            res.push_back(cur);
        }
        return res;
    }
};