How to determine, whether an integer array has duplicate elements or not?-CodePudding

I was asked to write a code, which indicates, whether an integer array has duplicate elements or not. This is what I have so far:

int[] input = {1, 2, 3, 4, 1};
boolean hasDuplicates = false;

for (int i = 0; i < input.length; i  ) {
    for (int j = i   1; j < input.length; j  ) {
        if (input[i] != input[j]) {
            hasDuplicates = false;
        } else if (input[i] == input[j]) {
            hasDuplicates = true;
        }
    }
}

However, this code does not work as it was intended to be, because hasDuplicates is always false.

CodePudding user response：

Once you have decided that the array hasDuplicates, you may never change that; you can't make disappear the duplicate after. Also use break the loop, at the moment hasDuplicates becomes true

int[] input = {1, 2, 3, 4, 1};
boolean hasDuplicates = false;

for (int i = 0; i < input.length; i  ) {
    for (int j = i   1; j < input.length; j  ) {
        if (input[i] == input[j]) {
            hasDuplicates = true;
            break;
        }
    }
    if (hasDuplicates)
        break;
}

Even easier in a method, directly use return

static boolean hasDup(int[] input) {
    for (int i = 0; i < input.length; i  )
        for (int j = i   1; j < input.length; j  )
            if (input[i] == input[j]) 
                return true;
    return false;
}

CodePudding user response：

You can also iterate over the array once if you use a HashSet. If the add() returns false then we know we have seen that number before.

int[] input = {1, 2, 3, 4, 1};
boolean hasDuplicates = false;
HashSet<Integer> duplicateSet = new HashSet<>();

for (int i = 0; i < input.length; i  ) {
    if(!duplicateSet.add(Integer.valueOf(input[i])))
    {
        hasDuplicates = true;
        break;
    }
}

return hasDuplicates;

CodePudding user response：

You can take the benefit of Set data-structure as well:

int[] input = {1, 2, 3, 4, 1};
boolean hasDuplicates = false;
    
Set<Integer> set = new HashSet<>(input.length);

for (int i : input) {
   if (set.add(i)) {
       hasDuplicates = false;
   }
   else {
      hasDuplicates = true;
      break;
   }
}

Set add(E e) returns true if this set did not already contain the specified element.

In a short way:

int[] input = {1, 2, 3, 4, 1};
boolean hasDuplicates = false;
        
Set<Integer> set = new HashSet<>(input.length);

for (int i : input) {
      if (!set.add(i)) {
           hasDuplicates = true;
           break;
     }
}

CodePudding user response：

Java 8 makes life easier. here you go

    int[] input = {1, 2, 3, 4, 1};
    boolean hasDuplicates = false;
    if(Arrays.stream(input).distinct().count() != input.length)
        hasDuplicates=true;
    System.out.println(hasDuplicates);