[LintCode] 596 Minimum Subtree 解题报告

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

Notice
LintCode will print the subtree which root is your return node.
It's guaranteed that there is only one subtree with minimum sum and the given binary tree is not an empty tree.


Example
Given a binary tree:

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

思路
这一类的题目都可以这样做：
开一个ResultType的变量result，来储存拥有最小sum的那个node的信息。
然后用分治法来遍历整棵树。
一个小弟找左子数的sum，一个小弟找右子树的sum。
同时，我们根据算出来的当前树的sum决定要不要更新result。
当遍历完整棵树的时候，result里记录的就是拥有最小sum的子树的信息。

Code

/**
 * 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
     */
    
    class ResultType {
        TreeNode node;
        int sum;
        public ResultType(TreeNode node, int sum) {
            this.node = node;
            this.sum = sum;
        }
    } 
    
    private ResultType result = null;
    
    public TreeNode findSubtree(TreeNode root) {
        // Write your code here
        if (root == null) {
            return null;
        }
        
        ResultType rootResult = helper(root);
        return result.node;
    }
    
    public ResultType helper(TreeNode root) {
        if (root == null) {
            return new ResultType(null, 0);
        }
        
        ResultType leftResult = helper(root.left);
        ResultType rightResult = helper(root.right);
        ResultType currResult = new ResultType(root, 
                leftResult.sum + rightResult.sum + root.val);
        
        if (result == null || currResult.sum < result.sum) {
            result = currResult;
        }
        
        return currResult;
    }
    
}