学步园

Recover
Binary Search Tree:

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) {}
 * };
 */
class Solution {
public:
    TreeNode* w1,*w2;
    void recoverTree(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        
		w1=NULL;
		w2=NULL;
		TreeNode* pre=NULL;
		inorder(root,pre);

		assert(w1&&w2);
		swap(w1->val,w2->val);
    }
	void inorder(TreeNode* root,TreeNode*& pre)
	{
		if (!root )
			return ;
		inorder(root->left,pre);
		if (pre&& pre->val>=root->val)
		{
			if (w1==NULL)
			{
				w1=pre;
				w2=root;
			}
			else
			{
				w2=root;
			}
		}
		pre=root;
		inorder(root->right,pre);
	}
};