LeetCode 344 Reverse String

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

Example 1:

Input: "hello"
Output: "olleh"

Example 2:

Input: "A man, a plan, a canal: Panama"
Output: "amanaP :lanac a ,nalp a ,nam A"

//方法一：用StringBuilder， Time： O(n), Space: O(n)
public String reverseString(String s) {
        if (s == null || s.length() == 0) {
            return s;
        }
        
        StringBuilder sb = new StringBuilder();
        
        for(int i = s.length() - 1; i >= 0; i--) {
            sb.append(s.charAt(i));
        }
        
        return sb.toString();
    }


//方法二：用array， Time： O(n), Space: O(n)
public String reverseString(String s) {
        if (s == null || s.length() == 0) {
            return s;
        }
        
        char[] result = s.toCharArray();
        int i = 0;
        int j = result.length - 1;
        
        while (i < j) {
            swap(i, j, result);
            i++;
            j--;
        }
        
        return new String(result);//最后要把array再转回string
    }
    
    public void swap(int i, int j, char[] c) {
        char temp = c[i];
        c[i] = c[j];
        c[j] = temp;
    }