Rotate an array of n elements to the right or left by k steps.

RotateArray.java

// rotate([1, 2, 3, 4, 5], 2, 'r')
// [5, 4, | 3, 2, 1] reverse(nums, 0, n-1)
// [4, 5, | 3, 2, 1] reverse(nums, 0, k-1)
// [4, 5, | 1, 2, 3] reverse(nums, k, n-1)

// rotate([1, 2, 3, 4, 5], 2, 'l')
// [5, 4, 3, | 2, 1] reverse(nums, 0, n-1)
// [3, 4, 5, | 2, 1] reverse(nums, 0, n-k-1)
// [3, 4, 5, | 1, 2] reverse(nums, n-k, n-1)

public void rotate(int[] nums, int k, char dir) {
	int n = nums.length;
	if(n == 0) return;
	k %= n;
	reverse(nums, 0, n-1);
	if(dir == 'r') {
		reverse(nums, 0, k-1);
		reverse(nums, k, n-1);
	} else if(dir == 'l') {
		reverse(nums, 0, n-k-1);
		reverse(nums, n-k, n-1);
	} else throw new IllegalArgumentException();
}

public void reverse(int[] a, int i, int j) {
	while(i < j) 
		swap(a, i++, j--);
}

public void swap(int[] a, int i, int j) {
	int temp = a[i];
	a[i] = a[j];
	a[j] = temp;
}