Problem statement

Write a function that reverses a string. The input string is given as an array of characters s.

You must do this by modifying the input array in-place with O(1) extra memory.

Example 1

Input: s = ['h','e','l','l','o']
Output: ['o','l','l','e','h']

Example 2

Input: s = ['H','a','n','n','a','h']
Output: ['h','a','n','n','a','H']

Constraints

• 1 <= s.length <= 10^5.
• s[i] is a printable ascii character.

Solution: Swap the opposites

Code

#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
void reverseString(vector<char>& s) {
    int i = 0;
    int j = s.size() - 1;
    while (i < j) {
        swap(s[i++], s[j--]);
    }
}
void printResult(vector<char>& s) {
    cout << "[";
    for (char c : s) {
        cout << c << ",";
    }
    cout << "]\n";
}
int main() {
    vector<char> s{'h','e','l','l','o'};
    reverseString(s);
    printResult(s);
    s = {'H','a','n','n','a','h'};
    reverseString(s);
    printResult(s);
}

Output:
[o,l,l,e,h,]
[h,a,n,n,a,H,]

Complexity

• Runtime: O(N), where N = s.length.
• Extra space: O(1).