subtask1 #2 TLE，求助

记搜

#include<bits/stdc++.h>
#define N 21

using namespace std;
typedef long long ll;

ll mem[N][N][N];

ll dfs(ll a, ll b, ll c){
    if (a <= 0 || b <= 0 || c <= 0) return 1;
    if (a > 20 || b > 20 || c > 20) return dfs(20,20,20);
    if (mem[a][b][c]) return mem[a][b][c];
    ll res = 0;
    if (a == 0 || b == 0 || c == 0) res = 1;
    //else if (a > 20 || b > 20 || c > 20) res = dfs(20,20,20);
    else if (a < b && b < c) res = dfs(a,b,c-1) + dfs(a,b-1,c-1) - \
        dfs(a,b-1,c);
    else res = dfs(a-1,b,c) + dfs(a-1,b-1,c) + dfs(a-1,b,c-1) - \
        dfs(a-1,b-1,c-1);
    if (a <= 20 && b <= 20 && c <= 20 && a > 0 && b > 0 && c > 0) \
        mem[a][b][c] = res;
    return res;
}

int main(){
    ll a,b,c;
    scanf("%lld%lld%lld", &a, &b, &c);
    memset(mem, 0, sizeof mem);
    while(a != -1 || b != -1 || c != -1){
        ll ans = dfs(a,b,c);
        printf("w(%lld, %lld, %lld) = %lld\n", a,b,c, ans);

        scanf("%lld%lld%lld", &a, &b, &c);
        memset(mem, 0, sizeof mem);
    }
    return 0;
}

dp

#include <bits/stdc++.h>
#define N 21

using namespace std;

typedef long long ll;
ll f[N][N][N];

int main(){
    ll res;
    ll a,b,c;
    scanf("%lld%lld%lld", &a, &b, &c);
    memset(f, 0, sizeof f);
    while(a != -1 || b != -1 || c != -1){
        for (int i = 0; i < N; i++)
            for (int j = 0; j < N; j++)
                for (int k = 0; k < N; k++){
                    if (i == 0 || j == 0 || k == 0) f[i][j][k] = 1;
                    else if (i<j && j<k) f[i][j][k] = f[i][j][k-1] + f[i][j-1][k-1] - f[i][j-1][k];
                    else f[i][j][k] = f[i-1][j][k] + f[i-1][j-1][k] + f[i-1][j][k-1] - f[i-1][j-1][k-1];
                }
        if (a <= 0 || b <= 0 || c <= 0) res = 1;
        else if (a > 20 || b > 20 || c > 20) res = f[20][20][20];
        else res = f[a][b][c];

        printf("w(%lld, %lld, %lld) = %lld\n", a,b,c, res);

        scanf("%lld%lld%lld", &a, &b, &c);
        memset(f, 0, sizeof f);
    }
    return 0;
}

都是subtask#1 的#2点TLE了，不知道为什么，有什么能优化的吗？谢谢指点