124. Binary Tree Maximum Path Sum

题目

Given a non-empty binary tree, find the maximum path sum.

For this problem, a path is defined as any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The path must contain at least one node and does not need to go through the root.

Example 1:

Input: [1,2,3]

       1
      / \
     2   3

Output: 6

Example 2:

Input: [-10,9,20,null,null,15,7]

   -10
   / \
  9  20
    /  \
   15   7

Output: 42

题目大意

给定一个非空二叉树，返回其最大路径和。本题中，路径被定义为一条从树中任意节点出发，达到任意节点的序列。该路径至少包含一个节点，且不一定经过根节点。

解题思路

• 给出一个二叉树，要求找一条路径使得路径的和是最大的。
• 这一题思路比较简单，递归维护最大值即可。不过需要比较的对象比较多。maxPathSum(root) = max(maxPathSum(root.Left), maxPathSum(root.Right), maxPathSumFrom(root.Left) (if>0) + maxPathSumFrom(root.Right) (if>0) + root.Val) ，其中，maxPathSumFrom(root) = max(maxPathSumFrom(root.Left), maxPathSumFrom(root.Right)) + root.Val

代码

package leetcode

import "math"

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func maxPathSum(root *TreeNode) int {
	if root == nil {
		return 0
	}
	max := math.MinInt32
	getPathSum(root, &max)
	return max
}

func getPathSum(root *TreeNode, maxSum *int) int {
	if root == nil {
		return math.MinInt32
	}
	left := getPathSum(root.Left, maxSum)
	right := getPathSum(root.Right, maxSum)

	currMax := max(max(left+root.Val, right+root.Val), root.Val)
	*maxSum = max(*maxSum, max(currMax, left+right+root.Val))
	return currMax
}

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