[Leetcode] Binary Tree Level Order Traversal II

Binary Tree Level Order Traversal II 题解

题目来源：https://leetcode.com/problems/binary-tree-level-order-traversal-ii/description/

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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).

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]
]

Solution

class Solution {
public:
    vector<vector<int> > levelOrderBottom(TreeNode* root) {
        vector<vector<int> > res;
        if (root == NULL)
            return res;
        queue<TreeNode*> q;
        q.push(root);
        q.push(NULL);
        vector<int> level;
        while (!q.empty()) {
            TreeNode *node = q.front();
            q.pop();
            if (node) {
                level.push_back(node -> val);
                if (node -> left)
                    q.push(node -> left);
                if (node -> right)
                    q.push(node -> right);
            } else {
                res.push_back(level);
                level.clear();
                if (!q.empty())
                    q.push(NULL);
            }
        }
        reverse(res.begin(), res.end());
        return res;
    }
};

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解题描述

这道题题意是，对二叉树进行层次遍历，但是要求是自底向上地遍历。上面给出的解法还是跟自顶向下的遍历基本相同，只是在返回结果之前对结果进行一个反转。