Recursion java assignment

public class RecursionLab {

public static void main(String args[]) {

System.out.println(reverseString("Hello world."));

System.out.println(sumDigits(3919));

System.out.println(sumPositives(new int[] {9, 0, -3, 5, 7, -6}, 0));

System.out.println(solvable1DMaze(new int[] {0, 1, 1, 0, 0, 1, 0}, 0, 2));

System.out.println(targetSum(new int[] {-51, -99, 88, -89, 34, 63, -100, 77, 43, 12}, 0, 0, 33));

}

private static String reverseString(String s) {

return null;

}

private static int sumDigits(int num) {

return 0;

}

private static int sumPositives(int arr[], int idx) {

return 0;

}

private static boolean solvable1DMaze(int maze[], int idx, int jump) {

return false;

}

private static boolean targetSum(int arr[], int idx, int sum, int target) {

if(idx >= arr.length) {

return sum == target;

}

return targetSum(arr, idx + 1, sum + arr[idx], target) ||

targetSum(arr, idx + 1, sum, target);

}