leetcode 653. 两数之和 IV - 输入 BST

/**
 * 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 {
    List<Integer> list = new ArrayList<>();
    public boolean findTarget(TreeNode root, int k) {
        find(root);
        int end = list.size() - 1;
        int st = 0;
        while (st < end) {
            int value = list.get(st) + list.get(end);
            if (value == k) {
                return true;
            } else if (value < k) {
                st++;
            } else {
                end--;
            }
        }
        return false;
    }

    public void find(TreeNode node) {
        if (node == null) {
            return;
        }
        find(node.left);
        list.add(node.val);
        find(node.right);
    }
}