124. Binary Tree Maximum Path Sum

Hard

A path in a binary tree is a sequence of nodes where each pair of adjacent nodes in the sequence has an edge connecting them. A node can only appear in the sequence at most once. Note that the path does not need to pass through the root.

The path sum of a path is the sum of the node's values in the path.

Given the root of a binary tree, return the maximum path sum of any non-empty path.

Example 1:

Input: root = [1,2,3]
Output: 6
Explanation: The optimal path is 2 -> 1 -> 3 with a path sum of 2 + 1 + 3 = 6.

Example 2:

Input: root = [-10,9,20,null,null,15,7]
Output: 42
Explanation: The optimal path is 15 -> 20 -> 7 with a path sum of 15 + 20 + 7 = 42.

Constraints:

• The number of nodes in the tree is in the range [1, 3 * 10^4].

• -1000 <= Node.val <= 1000

解題

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func maxPathSum(root *TreeNode) int {
    ans := math.MinInt32

    var helper func(*TreeNode)int
    helper = func(root *TreeNode) int {
        if root == nil {
            return 0
        }

        left := max(helper(root.Left), 0)
        right := max(helper(root.Right), 0)
        ans = max(ans, left + right + root.Val)

        return max(left, right) + root.Val
    }

    helper(root)

    return ans
}

func max(a, b int) int {
    if a > b {
        return a
    }
    return b
}