Leetcode 107. Binary Tree Level Order Traversal II

最新推荐文章于 2020-01-03 13:57:10 发布

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本文介绍了一种从叶节点到根节点的层次遍历二叉树的方法，并提供了一个具体的C++实现示例。该算法首先通过递归方式获取各层级的节点值，然后反转结果以实现从底部向顶部的层次遍历。

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],

return its bottom-up level order traversal as:

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

/**
 * 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:
    void check(TreeNode*a,vector<vector<int>>&ans,int dep){
        if(a == NULL) return ;
        if(ans.size() <= dep){
           vector<int> ax;
            ans.push_back(ax);
        }
        ans[dep].push_back(a->val);
        check(a->left,ans,dep+1);
        check(a->right,ans,dep+1);
    }
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        vector<vector<int>> ans;
        check(root,ans,0);
        for(int i = 0;i < ans.size() / 2; i++){
            swap(ans[i],ans[ans.size()-1-i]);
        }
        return ans;
    }
};