lintcode596- Minimum Subtree- easy

/**
 * 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;
 *     }
 * }
 */


public class Solution {
    /*
     * @param root: the root of binary tree
     * @return: the root of the minimum subtree
     */
     
    private int minSum = Integer.MAX_VALUE;
    private TreeNode minRoot = null;
    
    public TreeNode findSubtree(TreeNode root) {
        // write your code here
        
        if (root == null) {
            return minRoot;
        }
        
        subtreeSum(root);
        return minRoot;
        
    }
    
    private int subtreeSum(TreeNode root) {
        
        if (root == null) {
            return 0;
        }
        
        int leftSum = subtreeSum(root.left);
        int rightSum = subtreeSum(root.right);
        
        int result = leftSum + rightSum + root.val;
        
        if(result < minSum) {
            minSum = result;
            minRoot = root;
        }
        
        return result;
    }
}