【leetcode】504. 七进制数

char * convertToBase7(int num){
    if (num==0) return "0";
    char* s = (char*)calloc(1001,sizeof(char));
    int pst=1000, flag=(num<0)?1 :0;
    num = abs(num);
    char buffer[10]={0};
    while(num){
        sprintf(buffer,"%d",num%7);
        s[--pst]=buffer[0];
        num/=7;
    }
    if(flag) s[--pst]='-';
    return s+pst;
}