[每日 LeetCode] 107. Binary Tree Level Order Traversal II

原文链接 [每日 LeetCode] 107. Binary Tree Level Order Traversal II

Description:

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]
]

思路：本题是[每日 LeetCode] 102. Binary Tree Level Order Traversal 变型，要求层序遍历时从下至上访问。基本的流程和 102 题是一样的，区别在于保存数组的方式，可以使用 vector.insert()方法进行插入，每层结点插入到数组最前面就好。

C++ 代码

/**
 * 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{{root}};
        while(!q.empty()){
             vector<int> oneLevel;
            for (int i=q.size(); i > 0; i--){
                TreeNode* t = q.front();
                q.pop();
                oneLevel.push_back(t->val);
                if (t->left)
                    q.push(t->left);
                if (t->right)
                    q.push(t->right);
            }
            res.insert(res.begin(),oneLevel);
        }
        return res;
    }
};

运行时间：12ms

运行内存：13.4M