[leetcode-653-Two Sum IV - Input is a BST]

Given a Binary Search Tree and a target number, return true if there exist two elements in the BST such that their sum is equal to the given target.

Example 1:

Input:
    5
   /   3   6
 / \   2   4   7

Target = 9

Output: True

Example 2:

Input:
    5
   /   3   6
 / \   2   4   7

Target = 28

Output: False

思路：

中序遍历二叉树，将节点值保存在数组里面。然后用从头和尾部遍历刚刚得到的有序数组。

void middle( TreeNode* root, vector< int >& a )
{
    if( root == NULL )  return;
    middle( root->left, a );
    a.push_back( root->val );
    middle( root->right, a );
}
bool findTarget(TreeNode* root, int k)
{
    vector< int > a;
    middle( root, a );

    int n = a.size();
    for( int i = 0, j = n-1; i < j;  )
    {
        if( a[i] + a[j] < k )
            i++;
        else if( a[i] + a[j] > k )
            j--;
        else
            return true;
    }
    return false;
}