题目描述：

Given a binary tree, find the subtree with minimum sum. Return the root of the subtree.

样例：

Given a binary tree:

     1
   /   \
 -5     2
 / \   /  \
0   2 -4  -5 
return the node 1.

代码实现：

/**
 * 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;
 *     }
 * }
 */
class ResultType {
    TreeNode node;
    public int sum;
    public ResultType(int sum) {
        this.sum = sum;
    }
}
public class Solution {
    /**
     * @param root the root of binary tree
     * @return the root of the minimum subtree
     */
    private TreeNode subtree = null;
    private int subtreeSum = Integer.MAX_VALUE;
    public TreeNode findSubtree(TreeNode root) {
        helper(root);
        return subtree;
    }
    private ResultType helper(TreeNode root) {
        if (root == null) {
            return new ResultType(0);
        }
        ResultType left = helper(root.left);
        ResultType right = helper(root.right);
        ResultType result = new ResultType(left.sum + right.sum + root.val);
        //int sum = helper(root.left) + helper(root.right) + root.val;
        if (result.sum < subtreeSum) {
            subtreeSum = result.sum;
            subtree = root;
        }
        return result;
    }
}