#include<iostream.h>

#include<string.h>//complexity-n/2

int main()
{
int i,l,l1;
char str[100];
cout<<"enter string:";
gets(str);
l=strlen(str);
if(l%2==0)
l1=(l/2-1);
else
l1=l/2;
for(i=0;i<=l1;i++)/*swap elements from 2 ends till u reach
middle part of array*/
{
char t=str[i];
str[i]=str[l-i-1];
str[l-i-1]=t;
}
str[l]=0;
cout<<"\n\nreversed string is:"<<str;
getch();
return 0;
}

#include<stdio.h>
#include<string.h>
main(int argc, char *argv[])
{
  char  *string = argv[1];
  int len = strlen(string);
  int i = 0;
  int j = len - 1;
  printf("string before is %s\n", string);
  printf("len is %d\n", len);
  while(i <= j)
  {
    *(string+i) += *(string+j);
    *(string+j) = *(string+i) - *(string+j);
    *(string+i) = *(string+i) - *(string+j);
    i++;
    j--;
    if(len % 2)
     if(i == j) break;
  }
  printf("reversed string is %s\n", string);
}

#include<stdio.h>

void reverse(char *);

void main()
{
	char str[]="Hello";

	reverse(str);
	printf("Reverse String is %s",str);
	
}

void reverse(char *p)
{

	char *q=p;
	while(*++q!='\0');
	q--;

	while(p<q)
	{
		*p=*p+*q;
		*q=*p-*q;
		*p=*p-*q;
		p++;
		q--;
	}

}

#inclue<stdio.h>
       #include<string.h>
       main(){
           char a[10],i;
           int len=1;
           printf(" Enter string ");
           fgets(a,9,stdin);
     
           len = strlen(a);
     
      
          for(i=0 ; i<(len/2) ; i++){
                  a[i]=a[i]+a[len-2];
                  a[len-2]=a[i]-a[len-2];
                  a[i]=a[i]-a[len-2];
                  len--;
          }
          printf("\n Reverse string with n/2 complexity
 %s",a);
          return 0;
      }

refer c book!
    or
contact me

It's all O(n). You're finding the length of the string,
which itself is an O(n) operation.
So, O(n + n/2) = O(n).

#include <iostream>
#include <string>
using std::string;
using std::cout;
using std::endl;
using std::cin;

int main()
{
    string s("abcdefghijklmnopqrstuvwxyz");
    string::size_type s_size = s.size();

    for (string::size_type x = 0; x != s_size; x++){

        if ( (s_size -x -1) > x ){

            s[x] ^= s[s_size - x -1];
            s[s_size - x -1] ^= s[x];
            s[x] ^= s[s_size - x -1];

        }else{
            break;
        }

    }

    cout << s << endl;
}

int i=0,len=strlen(str);
int j=len/2;len--;
while(i<j)
{
*(str+i)^=*(str+len)^=*(str+i)^=*(str+len);
len--;i++;
}