Thursday, March 11, 2021, the weather is fine [Do not lament the past, do not waste the present, do not fear the future]
Contents of this article
1. Introduction
Sword refers to Offer 36. Binary search tree and doubly linked list
2. Problem solution (inspect the in-order traversal, need to store the previous node)
The middle order traverses each node and stores the previous node at the same time to complete the construction of the doubly linked list.
2.1 Recursion
class Solution {
public:
Node* pre, *head;
Node* treeToDoublyList(Node* root) {
if(root==nullptr) return nullptr;
dfs(root);
// 要求是双向循环链表，所以接上头和尾
head->left = pre;
pre->right = head;
return head;
}
void dfs(Node* cur){
if(cur==nullptr) return;
dfs(cur->left);
// 核心操作在这里
if(pre!=nullptr) pre->right = cur;
else head = cur;
cur->left = pre;
pre = cur;
dfs(cur->right);
}
};
2.2 Iteration
class Solution {
public:
Node* treeToDoublyList(Node* root) {
if(root==nullptr) return root;
stack<Node*> st;
Node* cur = root, *pre = nullptr, *head = nullptr;
while(cur || !st.empty()){
if(cur){
st.push(cur);
cur = cur->left;
}
else{
cur = st.top();
st.pop();
if(pre!=nullptr) pre->right = cur;
else head = cur;
cur->left = pre;
pre = cur;
cur = cur->right;
}
}
head->left = pre;
pre->right = head;
return head;
}
};
references
"Sword Finger Offer Second Edition"