首发于 语雀文档@blueju

    重点：

    代码：

    1. /**
    2.  * Definition for a binary tree node.
    3.  * class TreeNode {
    4.  *     val: number
    5.  *     left: TreeNode | null
    6.  *     right: TreeNode | null
    7.  *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
    8.  *         this.val = (val===undefined ? 0 : val)
    9.  *         this.left = (left===undefined ? null : left)
    10.  *         this.right = (right===undefined ? null : right)
    11.  *     }
    12.  * }
    13.  */
    15. function diameterOfBinaryTree(root: TreeNode | null): number {
    16.     if (root === null) return 0
    17.     let ans = 1
    18.     function depth(root: TreeNode | null) {
    19.         if (root === null) return 0
    20.         const l = depth(root.left)
    21.         const r = depth(root.right)
    22.         ans = Math.max(ans, l + r + 1)
    23.         return Math.max(l, r) + 1
    24.     }
    25.     depth(root)
    26.     return ans - 1
    27. };