ContinuousSubarraySum523.java

/**
 * Given a list of non-negative numbers and a target integer k, write a
 * function to check if the array has a continuous subarray of size at least 2
 * that sums up to the multiple of k, that is, sums up to n*k where n is also
 * an integer.
 * 
 * Example 1:
 * Input: [23, 2, 4, 6, 7],  k=6
 * Output: True
 * Explanation: Because [2, 4] is a continuous subarray of size 2 and sums up
 * to 6.
 * 
 * Example 2:
 * Input: [23, 2, 6, 4, 7],  k=6
 * Output: True
 * Explanation: Because [23, 2, 6, 4, 7] is an continuous subarray of size 5
 * and sums up to 42.
 * 
 * Note:
 * The length of the array won't exceed 10,000.
 * You may assume the sum of all the numbers is in the range of a signed
 * 32-bit integer.
 * 
 */

public class ContinuousSubarraySum523 {
    public boolean checkSubarraySum(int[] nums, int k) {
        if (k == 0) return kIsZero(nums);
        return kIsNotZero(nums, k);
    }
    
    public boolean kIsNotZero(int[] nums, int k) {
        Map<Integer, Integer> map = new HashMap<>();
        map.put(0, -1);
        int sum = 0;
        for (int i=0; i<nums.length; i++) {
            sum += nums[i];
            int mod = sum % k;
            if (map.containsKey(mod)) {
                if (i - map.get(mod) >= 2) return true;
            } else {
                map.put(mod, i);
            }
        }
        return false;
    }
    
    public boolean kIsZero(int[] nums) {
        int count = 0;
        for (int i=0; i<nums.length; i++) {
            if (nums[i] == 0) {
                count++;
                if (count >= 2) return true;
            } else {
                count = 0;
            }
        }
        return false;
    }


    /**
     * https://leetcode.com/problems/continuous-subarray-sum/solution/
     */
    public boolean checkSubarraySum2(int[] nums, int k) {
        int sum = 0;
        HashMap < Integer, Integer > map = new HashMap < > ();
        map.put(0, -1);
        for (int i = 0; i < nums.length; i++) {
            sum += nums[i];
            if (k != 0)
                sum = sum % k;
            if (map.containsKey(sum)) {
                if (i - map.get(sum) > 1)
                    return true;
            } else
                map.put(sum, i);
        }
        return false;
    }


}