【Leetcode】Recover Binary Search Tree

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题目链接：https://leetcode.com/problems/recover-binary-search-tree/

题目：

Two elements of a binary search tree (BST) are swapped by mistake.

Recover the tree without changing its structure.

思路：

1.中序 保存所有结点  空间复杂度O(n)

2.中序递归 保留前一个结点的指针 找到不对的结点

算法：

public void recoverTree(TreeNode root) {  
        inorder(root);  
        if (third != null) {// 二次逆序  
            int tmp = first.val;  
            first.val = third.val;  
            third.val = tmp;  
        } else {// 一次逆序  
            int tmp = second.val;  
            second.val = first.val;  
            first.val = tmp;  
        }  
    }  
  
    TreeNode pre, first, second, third;  
    public void inorder(TreeNode root) {  
        if (root == null)  
            return;  
        inorder(root.left);  
        if (pre == null) {  
            pre = root;  
        } else {  
            if (root.val < pre.val) {  
                if (first == null) { // 如果第一次逆序 需要交换first和second结点  
                    first = pre;  
                    second = root;  
                } else {// 如果第二次逆序 只需要交换first和third结点  
                    third = root;  
                }  
            }  
        }  
        pre = root;  
        inorder(root.right);  
    }  

【Leetcode】Recover Binary Search Tree

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原文地址：http://blog.csdn.net/yeqiuzs/article/details/51534172