LeetCode HOT100 - 二叉树的层序遍历

就是 bfs 的实现

每次 queue 中存在的就是这层的元素，所以要预先记录 size

然后遍历到这个元素后把它下一层的元素加进去

/**
 * 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) {
        queue<TreeNode*> q;
        vector<vector<int>> ans;
        if (!root) {
            return ans;
        }
        q.emplace(root);
        while (!q.empty()) {
            int n = q.size();
            vector<int> tmp(n);
            for (int i = 0; i < n; i++) {
                auto u = q.front();
                q.pop();
                tmp[i] = u->val;
                if (u->left) q.emplace(u->left);
                if (u->right) q.emplace(u->right);
            }
            ans.emplace_back(tmp);
        }
        return ans;
    }
};