Reverse String

• LeetCode: Reverse String
• Problem:

Write a function that takes a string as input and returns the string reversed.

• Example:

Given s = "hello", return "olleh".

• Analysis:

1. Use built-in reverse function.
2. Reverse it by a loop.

• Code - Java:

class Solution {
    public String reverseString(String s) {
        if (s == null || s.length() == 0) {
            return s;
        }
        int len = s.length();
        StringBuilder builder = new StringBuilder();
        for (int i = len - 1; i >= 0; i--) {
            builder.append(s.charAt(i));
        }
        return builder.toString();
    }
}

• Code - C++:

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

• Code - Python:

class Solution(object):
    def reverseString(self, s):
        """
        :type s: str
        :rtype: str
        """
        if s == None or len(s) == 0:
            return s
        return s[::-1]

• Time Complexity: O(N), N is the length of the input string
• Space Complexity: O(N), N is the length of the input string