102.二叉树的层序遍历.cpp

/*
 * @lc app=leetcode.cn id=102 lang=cpp
 *
 * [102] 二叉树的层序遍历
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if (!root) return res;

        queue<TreeNode*> q;
        q.push(root);
        while (q.size()) {
            vector<int> level;
            int len = q.size();
            while (len--) {
                auto x = q.front();
                q.pop();
                level.push_back(x->val);
                if (x->left) q.push(x->left);
                if (x->right) q.push(x->right);
            }
            res.push_back(level);
        }

        return res;
    }
};
// @lc code=end