lintcode94- Binary Tree Maximum Path Sum- medium

/**
 * 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: An integer
     */
     
    private class ResultType{
        public int wholeMax;
        public int linkedMax;
        public ResultType(int wholeMax, int linkedMax) {
            this.wholeMax = wholeMax;
            this.linkedMax = linkedMax;
        }
    }
    
    public int maxPathSum(TreeNode root) {
        // write your code here
        ResultType result = helper(root);
        return result.wholeMax;
    }
    
    private ResultType helper(TreeNode root) {
        
        if (root == null) {
            return new ResultType(Integer.MIN_VALUE, Integer.MIN_VALUE);
        }
        
        ResultType left = helper(root.left);
        ResultType right = helper(root.right);
        
        int linkedMax;
        int wholeMax;
        
        //千万小心因为你用的警示符是Integer.MIN_VALUE,所以你不能随便让答案加，要用0下限来限制，避免溢出。
        //如 linkedMax = Math.max (root.val, root.val + left.linkedMax)看似逻辑通顺，但仔细看就发现可能产生溢出错误！！
        linkedMax = root.val + Math.max(Math.max(0, left.linkedMax), right.linkedMax);
        
        int cross = root.val;
        cross += Math.max(0, left.linkedMax);
        cross += Math.max(0, right.linkedMax);
        wholeMax = Math.max(linkedMax, cross);
        wholeMax = Math.max(wholeMax, left.wholeMax);
        wholeMax = Math.max(wholeMax, right.wholeMax);
        
        return new ResultType(wholeMax, linkedMax);
    }
}