50 DSA-Questions: #3, #4

#3. Contains Duplicate

Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.

public class Exercise3 {
    private final Integer[] NUMS = {5,2,21,8,4,12,9,19,1,13};

    private void solve() {
        Map<Integer, Integer> map = new HashMap<>();
        for (int element : Arrays.asList(NUMS)) {
            if (map.containsKey(element)) {
                System.out.println("True");
                return;
            }
            map.put(element, 0);
        }
        System.out.println("False");
    }

    public static void main(String args[]) {
        new Exercise3().solve();
    }
}

#4. Product of Array Except Self

Given an integer array nums, return an array answer such that answer[i] is equal to the product of all the elements of nums except nums[I]. The product of any prefix or suffix of nums is guaranteed to fit in a 32-bit integer. You must write an algorithm that runs in O(n) time and without using the division operation.

public class Exercise4 {
    // private final Integer[] NUMS = {1,2,3,4};
    // private final Integer[] NUMS = {2,5,4,3,5};
    private final Integer[] NUMS = {0};

    private void solve() {
        if (NUMS.length <= 1) {
            System.out.println("Result N/A");
            return;
        }

        Integer[] leftToRight = new Integer[NUMS.length];
        leftToRight[0] = 1;
        leftToRight[1] = NUMS[0];
        for (int i=2; i<NUMS.length; i++) {
            leftToRight[i] = leftToRight[i-1] * NUMS[i-1];
        }

        Integer[] rightToLeft = new Integer[NUMS.length];
        rightToLeft[NUMS.length-1] = 1;
        rightToLeft[NUMS.length-2] = NUMS[NUMS.length-1];
        for (int i=NUMS.length-3; i>=0; i--) {
            rightToLeft[i] = rightToLeft[i+1] * NUMS[i+1];
        }

        Integer[] result = new Integer[NUMS.length];
        for (int i=0; i<NUMS.length; i++) {
            result[i] = leftToRight[i] * rightToLeft[i];
        }
        System.out.println("Result = " + Arrays.asList(result));
        // O(n)
    }

    public static void main(String args[]) {
        new Exercise4().solve();
    }
}