107-binary-tree-level-order-traversal-ii.md

107. Binary Tree Level Order Traversal II - 二叉树的层次遍历 II

给定一个二叉树，返回其节点值自底向上的层次遍历。 （即按从叶子节点所在层到根节点所在的层，逐层从左向右遍历）

例如：
给定二叉树 [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

返回其自底向上的层次遍历为：

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

---

题目标签：Tree / Breadth-first Search

题目链接：LeetCode / LeetCode中国

题解

Language	Runtime	Memory
cpp	8 ms	N/A

/**
 * 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) {
        vector<vector<int>> res;
        if(!root){
            return res;
        }
        queue<TreeNode*> seq;
        set<TreeNode*> vt;
        vt.insert(root);
        seq.push(root);
        while(!seq.empty()){
            int cnt = seq.size();
            vector<int> row;
            while(cnt--){
                TreeNode* tmp = seq.front();
                seq.pop();
                row.push_back(tmp->val);
                if(tmp->left && !vt.count(tmp->left)){
                    vt.insert(tmp->left);
                    seq.push(tmp->left);
                }
                if(tmp->right && !vt.count(tmp->right)){
                    vt.insert(tmp->right);
                    seq.push(tmp->right);
                }
            }
            res.push_back(row);
        }
        reverse(res.begin(), res.end());
        return res;
    }
};