模拟即可。

void solve()
{
    int t[] = {
        0,1,5,999,999,2,999,999,8,999
    }; // transform array
    
    string s;cin>>s;
    
    int h = t[s[4]-'0']*10 + t[s[3]-'0'], 
        m = t[s[1]-'0']*10 + t[s[0]-'0'];
    
    if(h>=0 && h<=23 && m>=0 && m<=59) 
        printf("%02d:%02d\n", h, m);
    else 
        printf("it's not a valid time\n");  
}