107. Binary Tree Level Order Traversal II*

107. Binary Tree Level Order Traversal II*

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

题目描述

Given a binary tree, return the bottom-up level order traversal of its nodes’ values. (ie, from left to right, level by level from leaf to root).

For example:

Given binary tree [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
return its bottom-up level order traversal as:
[
  [15,7],
  [9,20],
  [3]
]

C++ 实现 1

层序遍历, 最后将结果 reverse.

/**
 * 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>> levelOrderBottom(TreeNode* root) {
        if (!root) return {};
        vector<vector<int>> res;
        queue<TreeNode*> q;
        q.push(root);
        while (!q.empty()) {
            auto size = q.size();
            vector<int> tmp;
            while (size --) {
                auto r = q.front();
                q.pop();
                tmp.push_back(r->val);
                if (r->left) q.push(r->left);
                if (r->right) q.push(r->right);
            }
            res.push_back(tmp);
        }
        std::reverse(res.begin(), res.end());
        return res;
    }
};