// 非递归写法
function f(n) {if (0 === n) {return 1;}let res = 1;for (let i = 1; i <= n; ++i) {res *= i;}return res;
}// 递归写法
function g(n) {if (0 === n) {return 1;}return n*g(n-1);
}// 动态规划写法
let dp = [];
function h(n) {if (n < 0 || n !== Math.floor(n)) {return undefined;}if (dp[n]) {return dp[n];}if (0 === n) {dp[n] = 1;return 1;}for (let i = dp.length; i <= n; ++i) {dp[i] = dp[i-1]*i;}return dp[n];
}