543. Diameter of Binary Tree

Easy

Given the root of a binary tree, return the length of the diameter of the tree.

The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

The length of a path between two nodes is represented by the number of edges between them.

Example 1:

Input: root = [1,2,3,4,5]
Output: 3
Explanation: 3 is the length of the path [4,2,1,3] or [5,2,1,3].

Example 2:

Input: root = [1,2]
Output: 1

Constraints:

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

• -100 <= Node.val <= 100

解題

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func diameterOfBinaryTree(root *TreeNode) int {
    ans := 0

    if root == nil {
        return ans
    }

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

        ld := helper(root.Left)
        rd := helper(root.Right)

        ans = max(ans, ld + rd)

        return max(ld, rd) + 1
    }

    helper(root)

    return ans
}

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