[LeetCode]Recover Binary Search Tree

最新推荐文章于 2018-03-28 11:34:00 发布

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本文介绍了一种通过中序遍历来修复二叉搜索树中两个被错误交换节点的方法。当发现顺序错误时，记录下第一个错误节点 e1 和第二个错误节点 e2，通过交换这两个节点的值来恢复树的正确状态。

struct TreeNode {
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution {
//the clue is that if we traverse the tree in inorder, then it must be sorted, so:
//1. the first error element must be the first element of the pair who breaks the order
//2. the second error element must be the second element of the pair who breaks the order
//note: there is a special case that the two element exist in one single error pair
public:
	void recoverTree(TreeNode *root) {
		// Start typing your C/C++ solution below
		// DO NOT write int main() function
		TreeNode* prev = NULL;
		TreeNode* e1 = NULL;
		TreeNode* e2 = NULL;
		GetErrorElement(root, e1, e2, prev);
		if(e1 && e2)
			swap(e1->val, e2->val);
	}

	void GetErrorElement( TreeNode * root, TreeNode*& e1, TreeNode*& e2, TreeNode*& prev ) 
	{
		//throw std::exception("The method or operation is not implemented.");
		if(!root)
			return;
		GetErrorElement(root->left, e1, e2, prev);
		if (prev && prev->val > root->val)
		{
			e2 = root;//second error element
			if(!e1)//first error element
				e1 = prev;
		}
		prev = root;
		GetErrorElement(root->right, e1, e2, prev);
	}
};

second time

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void recorverTreeUtil(TreeNode* root, TreeNode*& prev, TreeNode*& first, TreeNode*& mid, TreeNode*& second)
    {
        if(root == NULL) return;
        recorverTreeUtil(root->left, prev, first, mid, second);
        if(prev != NULL && prev->val > root->val)
        {
            if(first == NULL) first = prev, mid = root;
            else second = root;
        }
        prev = root;
        recorverTreeUtil(root->right, prev, first, mid, second);
    }
    void recoverTree(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        TreeNode* first = NULL;
        TreeNode* mid = NULL;
        TreeNode* second = NULL;
        TreeNode* prev = NULL;
        recorverTreeUtil(root, prev, first, mid, second);
        if(second != NULL) swap(first->val, second->val);
        else swap(first->val, mid->val);
    }
};