【LeetCode】 Recover Binary Search Tree BST 中序遍历

题目：Recover Binary Search Tree

<span style="font-size:18px;">/*
 * LeetCode: recover the binary search tree
 * 题目：二叉树中有两个节点被交换了位置，找出它们，并且将它们换回来，要求用o(n)的连续空间
 * 知识点：1、BST树的特点：中序遍历后的节点的排列是按照非降的顺序
 * 思路：按照特点中序遍历，当遇到逆序的节点则按照保存相关节点，注意分为，交换的两个点是否相邻的两种情况
 */
package javaTrain;

public class Train6 {
	public TreeNode firstNode = null,preNode = null,secondNode = null;
	public int preVal = Integer.MIN_VALUE;
    public void recoverTree(TreeNode root) {
        if(root == null) return;
        findInChild(root); 

        int temp = firstNode.val;
       	firstNode.val = secondNode.val;
       	secondNode.val = temp;
        return;
    }
    private void findInChild(TreeNode root){
    	if(root == null) return;
    	findInChild(root.left);
    	if(root.val < preVal){
    		if(firstNode == null){
    			firstNode = preNode;
    			secondNode = root;
    		}
    		else{
    			secondNode = root;
    			return;
    		}
    	}
    	preNode = root;
    	preVal = root.val;
    	findInChild(root.right);
    }
}  </span>