建议先过一遍:今天是二叉树~-CSDN博客,very重要!
第一题:226. 翻转二叉树 - 力扣(LeetCode)
回溯写法:(前中后位置交换都可以)
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
void traverse(TreeNode* root){
if(root==nullptr){return;}
//前
TreeNode* temp=root->left;
root->left=root->right;
root->right=temp;
traverse(root->left);
//中
traverse(root->right);
//后
}
TreeNode* invertTree(TreeNode* root) {
traverse(root);
return root;
}
};
动规写法:(后序位置闷声干大事--交换)
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
TreeNode* invertTree(TreeNode* root) {
if(root==nullptr){return nullptr;}
//前
TreeNode* left=invertTree(root->left);
//中
TreeNode* right=invertTree(root->right);
//后
root->left=right;
root->right=left;
return root;
}
};
第二题:116. 填充每个节点的下一个右侧节点指针 - 力扣(LeetCode)
第一眼是层次遍历,但是不让用队列,,
回溯写法:(感觉很难想,将相邻节点之间的空隙看做一个整体)
/*
// Definition for a Node.
class Node {
public:
int val;
Node* left;
Node* right;
Node* next;
Node() : val(0), left(NULL), right(NULL), next(NULL) {}
Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}
Node(int _val, Node* _left, Node* _right, Node* _next)
: val(_val), left(_left), right(_right), next(_next) {}
};
*/
class Solution {
public:
void traverse(Node* left,Node* right){
if(left==nullptr||right==nullptr){return;}
//前
left->next=right;
traverse(left->left,left->right);
traverse(right->left,right->right);
traverse(left->right,right->left);
}
Node* connect(Node* root) {
if(root==nullptr){return nullptr;}
traverse(root->left,root->right);
return root;
}
};