每日一道Leetcode - 653. 两数之和 IV - 输入 BST【中序遍历】

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    
    
    public boolean findTarget(TreeNode root, int k) {
    
    
        // 定义一个列表存放中序遍历的值
        List<Integer> list = new ArrayList();
        // 中序遍历
        inorder(root,list);
        // 定义双指针于列表的左端喝右端
        // 中序遍历列表从小到大排列
        int l = 0, r = list.size()-1;
        while(l<r){
    
    
            int sum = list.get(l)+list.get(r);
            if(sum==k)
                return true;
            if(sum<k)
                l++;
            else
                r--;
        }
        return false;
    }
    public void inorder(TreeNode root, List<Integer> list){
    
    
        if(root==null) return;
        inorder(root.left,list);
        list.add(root.val);
        inorder(root.right,list);
    }
}