How to Reverse String in C++?

Write a method that reverses the given input string. Example: Given s = “12345”, return “54321”.

STL

The C++ STL provides the reverse method that does the job.

class Solution {
public:
    string reverseString(string s) {
        reverse(s.begin(), s.end());
        return s;
    }
};

The string constructor takes the iterator so we can use rbegin() and rend() in which the ‘r’ means reversed.

class Solution {
public:
    string reverseString(string s) {
        auto x = string(s.rbegin(), s.rend());
        return x;
    }
};

Two Pointers

We initialize two pointers, from the start and end respectively. Swap two characters until these pointers meet in the middle. O(n) complexity.

class Solution {
public:
    string reverseString(string s) {
        int i = 0, j = s.length() - 1;
        while (i < j) {
            swap(s[i++], s[j--]);
        }
        return s;
    }
};

Make a Copy

If we make a copy, we can re-assign the reverse characters easily. O(n) complexity.

class Solution {
public:
    string reverseString(string s) {
        string x = s;
        for (int i = 0; i < s.length(); i ++) {
            x[i] = s[s.length() - i - 1];
        }
        return x;
    }
};

Add Character Reverse

Another way is to add character in the reverse order.

class Solution {
public:
    string reverseString(string s) {
        string x = "";
        for (int i = 0; i < s.length(); i ++) {
            x = s[i] + x;
        }
        return x;
    }
};

See also: Reverse a Sentence (Sentence Reversal Algorithm)

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