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);
        }
    }
};