LintCode 二叉树中的最大路径和题目分析代码

/**
 * 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.
     */
     

public int maxPathSum(TreeNode root) {
        if(root == null)
            return 0;
        ArrayList<Integer> res = new ArrayList<>();
        res.add(Integer.MIN_VALUE);
        helper(root, res);
        return res.get(0);
    }
    
    private int helper(TreeNode root, ArrayList<Integer> res) {
        if(root == null)
            return 0;
        int left = helper(root.left,res);
        int right = helper(root.right,res);
        
        int cur = root.val + (left>0?left:0) + (right>0?right:0);
        
        if(cur>res.get(0))
            res.set(0, cur);
        
        return root.val + Math.max(left, Math.max(right, 0));
    }
    
}