1.2

reversesString.java

/**
 * Created by Readman on 6/15/14.
 */
public class reversesString {
    /*
    time: n
    space: 1
    
    by the way, i have no idea about the time complexity of valueof method. if someone know, plz msg me. thx 
    * */
    public static String reverses (char[] str) {
        int i = 0;
        int j = str.length - 1;
        while (i < j) {
            if (str[i] != str[j]) {
                swap(str, i , j);
            }
            i ++;
            j --;
        }
        return String.valueOf(str);
    }
    public static void swap (char[] str, int i, int j) {
        char temp = str[i];
        str[i] = str[j];
        str[j] = temp;
    }

    public static void main(String[] args) {
        String s = "abc";
        System.out.println(reverses(s.toCharArray()));
    }
}

I think the Space Complexity is also O(n), because the toCharArray() will traverse all the character in the String.