由於數組進行排序,如在評論中提到的,你可以執行2個二進制搜索找到數組中的最低索引,其中數字出現,其中出現次數最高的指數。添加二進制搜索以找到某些索引,並獲得O(log n)算法。
嘗試此代碼不與一些不同的陣列值。
public static void main(final String[] args) {
final int numberToCount = 7;
final int[] x = new int[]{1,2,3,4,4,6,6,6,6,7,7,7,7,7,8,8,8,8,8,8};
final int indexOfKnownOccurence = Arrays.binarySearch(x, numberToCount);
if (indexOfKnownOccurence < 0) {
System.out.println("No instances of the number found");
return;
}
final int lowerBound = findIndexOfFirstOccurence(x, numberToCount, 0, indexOfKnownOccurence);
final int upperBound = findIndexOfLastOccurence(x, numberToCount, indexOfKnownOccurence, x.length - 1);
System.out.println("Lower bound: " + lowerBound);
System.out.println("Upper bound: " + upperBound);
System.out.println("Number of occurrences: " + (upperBound - lowerBound + 1));
}
//Binary search for start index
public static int findIndexOfFirstOccurence(final int[] x, final int numberToFind, final int startIndex, final int endIndex) {
if (startIndex == endIndex) {
return startIndex;
} else if (x[startIndex] == numberToFind) {
return startIndex;
} else if (startIndex + 1 == endIndex) {
return endIndex;
}
final int midIndex = startIndex + (int)Math.floor((endIndex - startIndex)/2);
if (x[midIndex] == numberToFind) {
return findIndexOfFirstOccurence(x, numberToFind, startIndex, midIndex);
} else {
return findIndexOfFirstOccurence(x, numberToFind, midIndex, endIndex);
}
}
//Binary search for end index
public static int findIndexOfLastOccurence(final int[] x, final int numberToFind, final int startIndex, final int endIndex) {
if (startIndex == endIndex) {
return endIndex;
} else if (x[endIndex] == numberToFind) {
return endIndex;
} else if (startIndex + 1 == endIndex) {
return startIndex;
}
final int midIndex = startIndex + (int)Math.floor((endIndex - startIndex)/2);
if (x[midIndex] == numberToFind) {
return findIndexOfLastOccurence(x, numberToFind, midIndex, endIndex);
} else {
return findIndexOfLastOccurence(x, numberToFind, startIndex, midIndex);
}
}
你的數組x未排序。您可以使用二進制搜索來查找數字的計數。 –
當然。二進制搜索n-1,二進制搜索n + 1,查找中間元素的數量。 –
你的方法給你O(n),而二進制方法在O(logn) – Keiwan