Leetcode: 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?

 

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

递归，本以为用递归会超时呢，居然没有。。。

有一点需要注意，只要当前子树中的左子树中的结点和根或者右子树中的值交换，换完之后都将是左子树中的最大值。右子树同理，是右子树中的最小值。

如果交换是左子树中的两个值交换，即交换不跨越当前根节点，就交给递归吧。

 

/**

 * 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* findLargestInLeft(TreeNode* root)

    {

        if(root==NULL) return NULL;

    	TreeNode* left = NULL;

		TreeNode* right = NULL;

		if(root->left)

			left = findLargestInLeft(root->left);

		if(root->right)

			right = findLargestInLeft(root->right);

		if(left && right)

		{

			TreeNode* largest = root->val > left->val ? root : left;

			largest = largest->val > right->val ? largest : right;

			return largest;

		}else if(left && right==NULL)

		{

			return root->val > left->val ? root : left;

		}else if(left==NULL && right)

		{

			return root->val > right->val ? root : right;

		}else

			return root;

    }

    TreeNode* findLeastInRight(TreeNode* root)

    {

        if(root ==NULL) return NULL;

        TreeNode* left = NULL;

		TreeNode* right = NULL;

		if(root->left != NULL)

			left = findLeastInRight(root->left);

		if(root->right)

			right = findLeastInRight(root->right);

		if(left && right)

		{

			TreeNode* least = root->val < left->val ? root : left;

			least = least->val < right->val ? least : right;

			return least;

		}else if(left && right == NULL)

		{

			return root->val < left->val ? root : left;

		}else if(left == NULL && right)

		{

			return root->val < right->val ? root : right;

		}else

			return root;

    }

    void recoverTree(TreeNode *root) {

        // Start typing your C/C++ solution below

        // DO NOT write int main() function

        if(root == NULL)return;

        TreeNode* left = findLargestInLeft(root->left);

        TreeNode* right = findLeastInRight(root->right);

        if(left && right){

            if(left->val > root->val && root->val > right->val)

				swap(left->val, right->val);

			else if(root->val < left->val && left->val < right->val)

				swap(left->val, root->val);

			else if(left->val < right->val && right->val < root->val)

				swap(root->val, right->val);

			else{

				recoverTree(root->left);

				recoverTree(root->right);

			}

        }else if(left != NULL && right==NULL){

			if(root->val < left->val)

				swap(root->val, left->val);

			else

				recoverTree(root->left);

        }else if(left==NULL && right!=NULL){

			if(root->val > right->val)

				swap(root->val, right->val);

			else

				recoverTree(root->right);

        }

    }

};