[LintCode] 596. Minimum Subtree

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

Example

Example 1:

Input:
{1,-5,2,1,2,-4,-5}
Output:1
Explanation:
The tree is look like this:
     1
   /   
 -5     2
 /    /  
1   2 -4  -5 
The sum of whole tree is minimum, so return the root.

Example 2:

Input:
{1}
Output:1
Explanation:
The tree is look like this:
   1
There is one and only one subtree in the tree. So we return 1.

/**
 * 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 TreeNode res = null;
    private int globalMin = Integer.MAX_VALUE;
    public TreeNode findSubtree(TreeNode root) {
        // write your code here
        helper(root);
        return res;
    }
    
    private int helper(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int left = helper(root.left);
        int right = helper(root.right);
        if (left + right + root.val < globalMin) {
            globalMin = left + right + root.val;
            res = root;
        }
        return left + right + root.val;
    }
}