算法简单题，吾辈重拳出击 - 第 N 个泰波那契数

输入：n = 4
输出：4
解释：
T_3 = 0 + 1 + 1 = 2
T_4 = 1 + 1 + 2 = 4

输入：n = 25
输出：1389537

var tribonacci = function(n) {
    let x = 0
    let y = 1
    let z = 1
    let res = 0
    if(n<2) return n
    if(n==2) return 1
    for(let i=3;i<=n;i++){
        res = x+y+z
        x = y
        y = z
        z = res
    }
    return res
};

var tribonacci = function(n) {
    let x = 0
    let y = 1
    let z = 1
    if(n<2) return n
    if(n==2) return 1
    let res =  tribonacci(n-1) + tribonacci(n-2) + tribonacci(n-3)
    return res
};