Leetcode 99. Recover Binary Search Tree (Hard) (cpp)
Tag: Tree, Depth-first Search
Difficulty: Hard
/*
99. Recover Binary Search Tree (Hard)
Two elements of a binary search tree (BST) are swapped by mistake.
Recover the tree without changing its structure.
Note:
A solution using O(n) space is pretty straight forward. Could you devise a constant space solution?
*/
/**
* 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:
void recoverTree(TreeNode* root) {
TreeNode* first = NULL, *second = NULL, *pre = new TreeNode(INT_MIN);
inOrder(root, first, second, pre);
swap(first->val, second->val);
return;
}
private:
void inOrder(TreeNode* root, TreeNode*& first, TreeNode*& second, TreeNode*& pre) {
if (root == NULL) {
return;
}
inOrder(root->left, first, second, pre);
if (pre->val >= root->val) {
if (first == NULL) {
first = pre;
}
if (first != NULL) {
second = root;
}
}
pre = root;
inOrder(root->right, first, second, pre);
}
};