543. Diameter of Binary Tree

/**
 *543. Diameter of Binary Tree
 * https://leetcode.com/problems/diameter-of-binary-tree/description/
 * */

class TreeNode(var `val`: Int) {
    var left: TreeNode? = null
    var right: TreeNode? = null
}

//一般树的题:
//时间复杂度:O(n),因为每个节点都要访问一次
//空间复杂度:O(h),如果用递归的话，需要入stack，所以根据树的高度定义stack的深度
class Solution {
    var answer = 0
    //每条边只能调用一次
    fun diameterOfBinaryTree(root: TreeNode?): Int {
        if (root == null) {
            return 0
        }
        //递归方式，非递归的可以试
        diameterHelp(root)
        return answer
    }

    private fun diameterHelp(root: TreeNode?): Int {
        if (root == null) {
            return -1
        }
        val l = diameterHelp(root.left) + 1
        val r = diameterHelp(root.right) + 1
        //
        answer = Math.max(answer, l + r)
        return Math.max(l, r)
    }
}