LeetCode | 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.

分析

根据对BST中序遍历结果的升序特性，可以找到逆序对，这里要注意两种情况：

1. 如果交换节点刚好在中序遍历时，是相邻的，那么只有一个逆序对；

2. 如果不相邻，就有两个逆序对

代码

public class RecoverBinarySearchTree {
	private TreeNode first;
	private TreeNode second;
	private TreeNode preNode;

	public void recoverTree(TreeNode root) {
		first = null;
		second = null;
		preNode = null;
		inorder(root);
		assert first != null && second != null;
		int temp = first.val;
		first.val = second.val;
		second.val = temp;
	}

	private void inorder(TreeNode root) {
		if (root == null) {
			return;
		}
		inorder(root.left);
		if (preNode != null && root.val < preNode.val) {
			if (first == null) {
				first = preNode;
			}
			second = root;
		}
		preNode = root;
		inorder(root.right);
	}
}

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LeetCode | Recover Binary Search Tree

原文：http://blog.csdn.net/perfect8886/article/details/20283487