NumberOfGoodParisMap.java

public int numIdenticalPairs(int[] nums) {
    if (nums == null || nums.length == 0) {
        return 0;
    }

    // key: int value; value: number of occurrence
    Map<Integer, Integer> lookup = new HashMap<>();
    int goodPairsCount = 0;
    for (int i : nums) {
        if (lookup.containsKey(i)) {
            goodPairsCount += lookup.get(i);
            lookup.put(i, lookup.get(i) + 1);
        } else {
            lookup.put(i, 1);
        }
    }

    return goodPairsCount;
}