include <stdio.h>

int f(int a, int b, int c, int n) { if(a<0 || b<0 || c<0) return 0; if(n==0) return 1; return f(a - 1, b, c, n - 1) + f(a, b - 1, c, n - 1) + f(a, b, c - 1, n - 1) ; // 填空 } int main() { printf("%d\n", f(1,1,1,2)); printf("%d\n", f(1,2,3,3)); return 0; }