LeetCode 107. Binary Tree Level Order Traversal II 树的BFS、DFS
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- Binary Tree Level Order Traversal II
- 题意
- 思路
- 代码
- BFS
- DFS
- Binary Tree Level Order Traversal II
107. Binary Tree Level Order Traversal II
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 7return its bottom-up level order traversal as:
[
[15,7],
[9,20],
[3]
]
题意
类比102. Binary Tree Level Order Traversal
思路
102. Binary Tree Level Order Traversal 的结果逆序输出,这里使用方向迭代器
无论是102还是107都可以使用BFS和DFS两种方向解决。
达到遍历整颗树的效果,重点是维护好各个结点所属层数的信息。
代码
BFS
//bfsclass Solution {public: vector<vector<int>> levelOrder(TreeNode* root) { vector<vector<int>> res; if(root == NULL) { return res; } //queue<int> que; //想清楚队列存储的数据,是树的结点入队 //观察返回信息,还需包含层级信息,所以结点入队还需维护其层级的信息 queue<pair<TreeNode*,int>> que; que.push(make_pair(root,0)); while(!que.empty()) { //取队头元素 TreeNode *node = que.front().first; int level = que.front().second; que.pop(); //需要注意当前节点所在的层,是否在res中已经存在 if(level == res.size()) //若相等就说明res还不包含level层,这个结点肯定在新的层中 res.push_back(vector<int>()); //创建一个新的层,来加载该层的结点数 res[level].push_back(node->val); if(node->left) { que.push(make_pair(node->left,level+1)); //当左结点存在,那么左结点的层数是当前结点层数+1 } if(node->right) { que.push(make_pair(node->right,level+1)); } } return vector<vector<int>>(res.rbegin(),res.rend()); }};
DFS
class Solution {public: vector<vector<int> > res; void DFS(TreeNode* root, int level) { if (root == NULL) return; if (level == res.size()) // The level does not exist in output { res.push_back(vector<int>()); // Create a new level } res[level].push_back(root->val); // Add the current value to its level DFS(root->left, level+1); // Go to the next level DFS(root->right,level+1); } vector<vector<int> > levelOrderBottom(TreeNode *root) { DFS(root, 0); return vector<vector<int> > (res.rbegin(), res.rend()); }};
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