1026. Maximum Difference Between Node and Ancestor

Medium

Given the root of a binary tree, find the maximum value v for which there exist different nodes a and b where v = |a.val - b.val| and a is an ancestor of b.

A node a is an ancestor of b if either: any child of a is equal to b or any child of a is an ancestor of b.

Example 1:

Input: root = [8,3,10,1,6,null,14,null,null,4,7,13]
Output: 7
Explanation: We have various ancestor-node differences, some of which are given below :
|8 - 3| = 5
|3 - 7| = 4
|8 - 1| = 7
|10 - 13| = 3
Among all possible differences, the maximum value of 7 is obtained by |8 - 1| = 7.

Example 2:

Input: root = [1,null,2,null,0,3]
Output: 3

Constraints:

• The number of nodes in the tree is in the range [2, 5000].

• 0 <= Node.val <= 10^5

解題

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

    var helper func(*TreeNode, int, int)
    helper = func(root *TreeNode, max int, min int) {
        if root == nil {
            return
        }

        if abs(max - root.Val) > ans {
            ans = abs(max - root.Val)
        }
        if abs(min - root.Val) > ans {
            ans = abs(min - root.Val)
        }

        nmax := max
        nmin := min
        if root.Val > nmax { nmax = root.Val }
        if root.Val < nmin { nmin = root.Val }

        helper(root.Right, nmax, nmin)
        helper(root.Left, nmax, nmin)
    }

    helper(root, root.Val, root.Val)

    return ans
}

func abs(a int) int {
    if a < 0 {
        return -a
    }
    return a
}