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 binary tree
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
struct TreeNode
{
int val;
TreeNode *left;
TreeNode *right;
TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
class Solution {
public:
void recoverTree(TreeNode *root) {
pre = n1 = n2 = NULL;
dfs(root);
n1->val ^= n2->val;
n2->val ^= n1->val;
n1->val ^= n2->val;
}
private:
TreeNode *pre,*n1,*n2;
void dfs(TreeNode *root)
{
if(root == NULL) return;
dfs(root->left);
if(pre != NULL &&pre->val > root->val)
{
n1 = root;
if(n2 == NULL) n2 = pre;
}
pre = root;
dfs(root->right);
}
};