344. Reverse String

class Solution {
public:
string reverseString(string s) {
int length=s.length();
if(length==0) return s;
char tmp;
for(int i=length-1,j=0;i>j;i--,j++) {
tmp=s[i];
s[i]=s[j];
s[j]=tmp;
}
return s;
}
};