// Problen 1
private static int[] twoSum(int[] arr, int target) {
    HashMap<Integer, Integer> visited = new HashMap<>();    // num -> idx
    for (int i=0; i< arr.length; i++) {
        int required = target-arr[i];
        if (visited.containsKey(required)) {
            return new int[] {visited.get(required), i};
        }
        visited.put(arr[i], i);
    }
    return new int[] {-1,-1};
}
    
// Problem 2    
private static int maxProfit(int[] arr) {
    int minIdx = minPrice(arr);
    int cost = arr[minIdx];

    int max = 0;
    for (int i=minIdx+1; i<arr.length; i++) {
        int profit = arr[i] - cost;
        if (profit > max) {
            max = profit;
        }
    }
    return max;
}

private static int minPrice(int[] arr) {
    int minIdx = 0;
    for (int i=1; i<arr.length; i++) {
        if (arr[i] < arr[minIdx]) {
            minIdx = i;
        }
    }
    return minIdx;
}