Algorithm to rotate an array in linear time

Question

How to rotate an integer array by i times using swap function only in linear time.

Answer 1

Here's a small snippet thats works in O(n), written in JavaScript. The keyconcept is, that you always have to work with the replaced item.

function swap(arr, a, v) {
    var old = arr[a];
    arr[a] = v;
    return old;
}

function rotate(arr, n) {
    var length = arr.length;
    n = n % length;
    if(!n) return arr;

    for(var cnt = 0, 
            index = 0,
            value = arr[index],
            startIndex = index; 
        cnt < length; 
        cnt++) {

        // Calc next index
        var nextIndex = mapIndex(index, n, length);

        // Swap value with next
        value = swap(arr, nextIndex, value)

        if(nextIndex == startIndex) {
            startIndex = index = mapIndex(index, 1, length);
            value = arr[index];
        } else {
            index = nextIndex;
        }
    }

    return arr;
}

function mapIndex(index, n, length) {
    return (index - n + length) % length;
}

console.log(rotate([1,2,3,4,5,6,7,8,9], 5))
console.log(rotate([1,2,3,4,5,6], 2))