653.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool findTarget(TreeNode* root, int k) {
        vector<int> nums;
        inorder(root, nums);
        int l = 0;
        int r = nums.size() - 1;
        while(l < r) {
            int sum = nums[l] + nums[r];
            if (sum == k) return true;
            else if (sum < k)
                ++l;
            else
                --r;
        }
        return false;
    }
private:
    void inorder(TreeNode* root, vector<int>& nums) {
        if (root == nullptr) return;
        inorder(root->left, nums);
        nums.push_back(root->val);
        inorder(root->right, nums);
    }
};