题解 | 判断是不是二叉搜索树

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类
     * @return bool布尔型
     */
    public boolean isValidBST (TreeNode root) {
        // write code here
        boolean[] flag = new boolean[] {true};
        inOrder(root, new int[1], new boolean[] {true}, flag);
        return flag[0];
    }

    public void inOrder(TreeNode root, int[] pre, boolean[] isFirst,
                        boolean[] flag) {
        if (root.left != null) inOrder(root.left, pre, isFirst, flag);
        if (isFirst[0]) {
            pre[0] = root.val;
            isFirst[0] = false;
        } else {
            if (pre[0] >= root.val) {
                flag[0] = false;
                return;
            }
            pre[0] = root.val;
        }
        if (root.right != null) inOrder(root.right, pre, isFirst, flag);
    }
}