描述
给出一棵二叉搜索树以及其中的一个节点，找到这个节点在这棵树中的中序前驱节点。
在线评测地址：领扣题库官网

样例1
输入: root = {2,1,3}, p = 1
输出: null

样例2
输入: root = {2,1}, p = 2
输出: 1

用while循环模拟递归

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: the given BST
     * @param p: the given node
     * @return: the in-order successor of the given node in the BST
     */
    public TreeNode inorderPredecessor(TreeNode root, TreeNode p) {
        // write your code here
        TreeNode succ = null;
        while (root != null) {
            if (p.val <= root.val) {
                root = root.left;
            } else {
                if(succ == null || root.val > succ.val){
                    succ = root;
                }
                root = root.right;
                
            }
        }
        return succ;        
    }
}

更多题解参考：九章官网solution