[leetcode]99. Recover Binary Search Tree@Java解题报告

https://leetcode.com/problems/recover-binary-search-tree/description/

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?

package go.jacob.day805;

public class Demo2 {
	//firstElem第一个被交换的element，secondElem被交换的element
	TreeNode firstElem = null, secondElem = null;
	//把上一个节点初始化为最小值
	TreeNode preElem = new TreeNode(Integer.MIN_VALUE);

	public void recoverTree(TreeNode root) {
		// 中序遍历
		inOrderTraverse(root);

		int tmp = firstElem.val;
		firstElem.val = secondElem.val;
		secondElem.val = tmp;
	}

	private void inOrderTraverse(TreeNode root) {
		if (root == null)
			return;
		inOrderTraverse(root.left);
		// 1234567到1264537
		if (firstElem == null && preElem.val >= root.val) {
			firstElem = preElem;
		}
		if (firstElem != null && preElem.val >= root.val) {
			secondElem = root;
		}

		preElem = root;
		inOrderTraverse(root.right);
	}

	private class TreeNode {
		int val;
		TreeNode left;
		TreeNode right;

		TreeNode(int x) {
			val = x;
		}
	}
}