123456789101112131415161718192021222324252627282930
function waysToStep(n: number): number {    return Number(waysToStep_bigint_mod(BigInt(n), 1000000007n));}
function waysToStep_bigint_mod(n: bigint, mod: bigint): bigint {    const cached = cache.get(n);    if (cache.has(n) && typeof cached !== "undefined") {        return cached;    }    for (let i = 0n; i <= n; i++) {        if (!cache.has(i)) {            const result =                ((cache.get(i - 1n) ?? waysToStep_bigint_mod(i - 1n, mod)) +                    (cache.get(i - 2n) ?? waysToStep_bigint_mod(i - 2n, mod)) +                    (cache.get(i - 3n) ?? waysToStep_bigint_mod(i - 3n, mod))) %                mod;            cache.set(i, result);        }    }    return waysToStep_bigint_mod(n, mod);}
const cache = new Map<bigint, bigint>([    [0n, 0n],    [1n, 1n],    [2n, 2n],    [3n, 4n],]);export default waysToStep;