The description of the problem

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 constant extra memory.
An example:
Input: s = [‘h’, ‘e’, ‘l’, ‘l’, ‘o’]
Output: [‘o’, ‘l’, ‘l’, ‘e’, ‘h’]

The intuition

1. two indexes, one of which starts from the begins, the other of which starts from the ends.

The solution code

The best one

#include 
#include 
using namespace std;
class Solution {
public:
    void reverseString(vector<char>& s) {
        int n = s.size();
        for (int i = 0, j = n - 1; i < n/2; i++, j--) {
            //swap(s[i], s[j]);
            // use xor operation to swap two elements
            s[i] ^= s[j];
            s[j] ^= s[i];
            s[i] ^= s[j];
        }
    }
};
int main() {
    Solution s;
    vector<char> s1 = {'h', 'e', 'l', 'l', 'o'};
    std::cout << "before reverse: ";
    for (auto c : s1) {
        std::cout << c << " ";
    }
    std::cout << std::endl;
    std::cout << "after  reverse: ";
    s.reverseString(s1);
    for (auto c : s1) {
        cout << c << " ";
    }
    cout << endl;
    return 0;
}
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The most intuitive one

#include 
#include 
using namespace std;
class Solution {
public:
    void reverseString(vector<char>& s) {
        int n = s.size();
        vector<char> original(s);
        for (int i = 0; i < n; i++) {
            s[i] = original[n - i - 1];
        }
    }
};
int main() {
    Solution s;
    vector<char> s1 = {'h', 'e', 'l', 'l', 'o'};
    std::cout << "before reverse: ";
    for (auto c : s1) {
        std::cout << c << " ";
    }
    std::cout << std::endl;
    std::cout << "after  reverse: ";
    s.reverseString(s1);
    for (auto c : s1) {
        cout << c << " ";
    }
    cout << endl;
    return 0;
}
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The corresponding results

before reverse: h e l l o 
after  reverse: o l l e h 
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