687. 最长同值路径（算法200题-34）

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
const longestUnivaluePath = function (root) {
    let res = 0
    const calculate = function (root) {
        if (root == null) return 0
        const leftCount = calculate(root.left)
        const rightCount = calculate(root.right)
        let left = 0,
            right = 0
        if (root.left != null && root.left.val == root.val) {
            left = leftCount + 1
        }
        if (root.right != null && root.right.val == root.val) {
            right = rightCount + 1
        }
        res = Math.max(res, left + right)
        return Math.max(left, right)
    }
    calculate(root)
    return res
};