LeetCode：124二叉树中的最大路径和

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    //记录最终的最大路径和，初始为极小值，防范全树都为负数的情况
    int maxSum = Integer.MIN_VALUE;
        
    public int maxPathSum(TreeNode root) {
        calculateSinglePath(root);
        return maxSum;
    }
    //计算当前节点能为父节点提供的最大路径和
    private int calculateSinglePath(TreeNode node){
        if(node == null){
            return 0;
        }
        //递归计算左右子树
        int leftProfit = Math.max(0,calculateSinglePath(node.left));
        int rightProfit = Math.max(0,calculateSinglePath(node.right));
        //倒V型的内部和
        int currentInternalSum = leftProfit + node.val + rightProfit;

        maxSum = Math.max(maxSum, currentInternalSum);

        return node.val + Math.max(leftProfit,rightProfit);
    }
}

maxSum用于记录全局最终的最大值；而函数返回值用于记录局部最大值，往上传递给父节点