SGU111 Big Data kaifang Evolution

Problem: You are given natural number X. Find such maximum integer number that it square is not greater than X. (X. length <1000)

Solution: I want to use java for water, but I kneel down. I found the formula on the Internet, but another short program is even more amazing. No =. = So it is included here.

Solution: A small piece of code can solve it but I don't know why. _ (: 3 "done )_

#include 
 
  #include 
  
   #include 
   
    using namespace std;int l;int work(int o,char *O,int I) {    char c, *D=O ;    if (o>0) {        for (l=0;D[l];D[l++]-=10) {            D[l++] -= 120;            D[l] -= 110;            while (!work(0,O,l)) D[l] += 20;            putchar((D[l]+1032)/20);        }        putchar(10);    }    else {        c = o+(D[I]+82)%10-(I>l/2)*(D[I-l+I]+72)/10-9;        D[I] += I<0?0:!(o=work(c/10,O,I-1))*((c+999)%10-(D[I]+92)%10);    }    return o;}int main() {    char s[1200];    s[0]='0';    scanf("%s",s+1);    if (strlen(s)%2 == 1) work(2,s+1,0);    else work(2,s,0);    return 0;}