leetcode-124:Binary Tree Maximum Path Sum(Java)

Binary Tree Maximum Path Sum

Given a binary tree, find the maximum path sum.

The path may start and end at any node in the tree.

For example:
Given the below binary tree,

要求：计算二叉树中的路径之和最大值，起止节点为任意

解法：动态规划，对每一个节点，以该节点为根节点的最大值设为value，其左子树的路径最大值为lmax，右子树的路径最大值为rmax，(!!! 注意：这里的lmax和rmax指的是从左/右子节点出发的某一条单向路径，例如对于节点4，lmax=2+3 rmax=6+7+8,而并不是lmax=1+2+3 rmax=5+6+7+8)那么有：

value = value + (lmax>0?lmax:0) + (rmax>0?rmax:0) ;

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.math.*;
public class Solution {
    int maxSum = Integer.MIN_VALUE;
    
    public int maxPathSum(TreeNode root) {
        if(root == null)
            return 0;
            
        getMaxSumWithCurNode(root);
        return maxSum;
    }
    
    
    int getMaxSumWithCurNode(TreeNode curNode){
        int lmax = 0, rmax = 0;
        int value = curNode.val; // 包含当前节点的最大路径和
        if(curNode.left != null){
            lmax = getMaxSumWithCurNode(curNode.left);
        }
        if(curNode.right != null){
            rmax = getMaxSumWithCurNode(curNode.right);
        }
        
        value = value + (lmax>0?lmax:0) + (rmax>0?rmax:0) ;
        if(value > maxSum)
            maxSum = value;
        // 注意这里的返回值，取左右子树其中一条路径    
        return curNode.val+Math.max( lmax>0?lmax:0, rmax>0?rmax:0 );
        
    }
}