為什麼代碼中的comp函數只需返回0就可以表示兩串相同？

#include&
#include&
#include& #include&
struct comp
{
bool operator() (const std::string a, const std::string b)
{
int l1=a.length(),l2=b.length();
if(l1!=l2) return l1↦
std::map&::iterator it_map;
std::multimap& &>multimap;
std::multimap&::iterator it_mm;
int a,b,n;std::string input,inp;
int main()
{
std::ios::sync_with_stdio(false);
std::cin&>&>a&>&>b&>&>n;
while(std::cin&>&>input)
inp+=input;
int l=inp.length();
for(int i=a;i&<=b;i++) for(int j=0;j+i&<=l;j++) map[inp.substr(j,i)]++; for(it_map=map.begin();it_map!=map.end();it_map++) multimap.insert(std::pair&(it_map-&>second,it_map-&>first));
int old=-1,thisout,totout=0,first;bool flag=0;
for(it_mm=multimap.begin();it_mm!=multimap.end();it_mm++)
{
first=it_mm-&>first;
if((first!=old old!=-1) || (thisout%6==0 thisout!=0)) {std::cout&<&<" ";flag=1;} if(!((first!=old old!=-1) || (thisout%6==0 thisout!=0)) old!=-1) {std::cout&<&<" ";flag=0;} if(first!=old) { totout++; if(totout==n+1) break; std::cout&<&second;
flag=0;
thisout++;
}
if(!flag) std::cout&<&