如何使 java 嵌套循环高效

Question

I currently need to code a program that does a pairwise comparison to find matching pairs in an int list, but the only way I can think of doing it is through a nested for loop. Is there a way to make the time complexity O(n) instead of the O(n^2) of a nested for loop?

int[] arr = new int[n];
int total = 0;
for (int i=0; i<n; i++){
    for (int j=i+1; j<n; j++){
        if (arr[i] == arr[j]){
            total++;
        }
    }
}

Answer 1

Looks like you are only worried about the count. So, if modifying an array is not an issue then, apply quick sort in O(nlog(n)) and then count the neighbours in O(n).

Answer 2

You can use a HashSet which has O(1) complexity of contains method - because hashCode of Integer value in Java is well distributed (it's just a value of the Integer ) you should have constant complexity always

HashSet<Integer> set = new HashSet();
for (int i=0; i<n; i++) {
    if(set.contains(arr[i])) {
        total++;
    } else {
        set.add(arr[i]);
    }
}

Read more here:

• HashSet look-up complexity?

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There's also one additional algorithm that could be implemented here but it require some data limitations (due to the max length of array on your machine).

You could just create an int array yourarray initialized with 0 with a length of max(arr) + 1 . Then you are iterating over arr every time executing yourarray[arr[i]]++ and then you are iterating over yourarray checking where the value is greater than 1 - if it this then it means that this value has to repeat

Answer 3

O(n) solution. Try this.

public static void main(String[] args) {
    int[] arr = {0, 1, 2, 3, 4, 3, 2, 3};
    int total = arr.length - (int)IntStream.of(arr).distinct().count();
    System.out.println(total);
}

output:

3

Answer 4

I think you can use a HashSet to resolve this problem. The HashSet in JAVA doesn't allow insert into a duplicate element. So you can use it to making the time complexity become O(n).

        int[] arr = new int[n];
        Set<Integer> set = new HashSet<Integer>();
        Integer total = 0;
        for (int x: arr)
        {
            if (!set.add(x))
            {
               total++;
            }
        }