an implementation of next_permutation() written in java

NextPermutation.java

public class NextPermutation {
	
	public static void swap(int[] arr, int index1, int index2) {
		int tmp = arr[index2];
		arr[index2] = arr[index1];
		arr[index1] = tmp;
	}
	
	public static boolean nextPermutation(int[] arr) {
		final int indexLast = arr.length - 1;
		
		// 1. Finding pivot
		int indexPivot = -1;
		for (int i = indexLast; i >= 1; --i) {
			if (arr[i-1] < arr[i]) {
				indexPivot = i - 1;
				break;
			}
		}
		if (indexPivot == -1) return false; // last perm
		
		// 2. Finding the one to be swapped + Swap
		int indexToBeSwapped = -1;
		for (int i = indexLast; i > indexPivot; --i) {
			if (arr[indexPivot] < arr[i]) {
				indexToBeSwapped = i;
				break;
			}
		}
		swap(arr, indexPivot, indexToBeSwapped);
		
		// 3. Reversing sequence of the rest
		final int nSwaps = (int)Math.floor((indexLast - indexPivot) / 2);
		int indexLeft = indexPivot + 1;
		int indexRight = indexLast;
		for (int i = 0; i < nSwaps; ++i, ++indexLeft, --indexRight) {
			swap(arr, indexLeft, indexRight);
		}
		return true;
	}
	
	public static void printArr(int[] arr) {
		StringBuilder sb = new StringBuilder();
		for (int e : arr) {
			sb.append(e);
			sb.append(' ');
		}
		System.out.println(sb);
	}
	
	public static void main(String[] args) {
		int[] arr = new int[5];
		for (int i = 0; i < 5; ++i) arr[i] = i;
		do {
			printArr(arr);
		} while (nextPermutation(arr));
	}

} // public class

PrevPermutation.java

public class PrevPermutation {
	
	public static void swap(int[] arr, int index1, int index2) {
		int tmp = arr[index2];
		arr[index2] = arr[index1];
		arr[index1] = tmp;
	}
	
	public static void swap(int[] arr, int i1, int i2) {
		int tmp = arr[i1];
		arr[i1] = arr[i2];
		arr[i2] = tmp;
	}

	public static boolean nextPermutation(int[] arr) {
		int indexPivot = -1;
		for (int i = arr.length - 1; i >= 1; --i ) {
			if (arr[i-1] < arr[i]) { // <---------------------------------------------------
				indexPivot = i - 1;
				break;
			}
		}
		if (indexPivot == -1) return false;
		
		// 2. 피벗과 한 놈 바꾸기
		int indexToSwap = -1;
		for (int i = arr.length - 1; i > indexPivot; --i ) {
			if (arr[indexPivot] < arr[i]) { // <---------------------------------------------------
				indexToSwap = i;
				break;
			}
		}
		swap(arr, indexPivot, indexToSwap);
		
		// 3. 나머지 역순
		for (int i = 0; i < Math.floor((arr.length - 1 - indexPivot) / 2); ++i) {
			swap(arr, indexPivot + 1 + i, arr.length - 1 - i);
		}
		return true;
	} // func
	
	public static boolean prevPermutation(int[] arr) {
		int indexPivot = -1;
		for (int i = arr.length - 1; i >= 1; --i ) {
			if (arr[i-1] > arr[i]) { // <---------------------------------------------------
				indexPivot = i - 1;
				break;
			}
		}
		if (indexPivot == -1) return false;
		
		// 2. 피벗과 한 놈 바꾸기
		int indexToSwap = -1;
		for (int i = arr.length - 1; i > indexPivot; --i ) {
			if (arr[indexPivot] > arr[i]) { // <---------------------------------------------------
				indexToSwap = i;
				break;
			}
		}
		swap(arr, indexPivot, indexToSwap);
		
		// 3. 나머지 역순
		for (int i = 0; i < Math.floor((arr.length - 1 - indexPivot) / 2); ++i) {
			swap(arr, indexPivot + 1 + i, arr.length - 1 - i);
		}
		return true;
	} // func
	
	public static void printArr(int[] arr) {
		StringBuilder sb = new StringBuilder();
		for (int e : arr) {
			sb.append(e);
			sb.append(' ');
		}
		System.out.println(sb);
	}
	
	public static void main(String[] args) {
		int[] arr = new int[5];
		for (int i = 0; i < 5; ++i) arr[i] = i;
		do {
			printArr(arr);
		} while (nextPermutation(arr));
    do {
			printArr(arr);
		} while (prevPermutation(arr));
	}

} // public class