Screenshot-2018-6-4 ATS ( ats5515) Twitter
”例の挿入、削除、ランダム取得がO(1)のset” を実装してみた。

source code

class IntSet {
private static final int EMPTY = -1;
private int size;
private int[] indexToValue;
private int[] valueToIndex;

public IntSet(int capacity) {
this.size = 0;
indexToValue = new int[capacity];
valueToIndex = new int[capacity];
Arrays.fill(valueToIndex, EMPTY);
}

public boolean add(int value) {
if (valueToIndex[value] != EMPTY) {
return false;
}
indexToValue[size] = value;
valueToIndex[indexToValue[size]] = size;
size++;
return true;
}

public boolean remove(int index) {
if (size == 0) {
return false;
}
assert index < size;
int swap = indexToValue[index];
indexToValue[index] = indexToValue[size - 1];
indexToValue[size - 1] = swap;

valueToIndex[indexToValue[index]] = index;
valueToIndex[indexToValue[size - 1]] = EMPTY;

size--;
return true;
}

public boolean removeValue(int value) {
int index = indexOf(value);
if (index == EMPTY) {
return false;
}
remove(index);
return true;
}

public int get(int index) {
assert index < size;
return indexToValue[index];
}

public int indexOf(int value) {
return valueToIndex[value];
}

public int size() {
return size;
}

public boolean isEmpty() {
return size() <= 0;
}

public void clear() {
for (; size() > 0;) {
remove(0);
}
}

public boolean contains(int value) {
return indexOf(value) != EMPTY;
}

}