Reorder array

最新推荐文章于 2021-02-24 20:30:40 发布

转载最新推荐文章于 2021-02-24 20:30:40 发布 · 324 阅读

本文介绍了一种特殊的数组重排序算法，该算法能在O(n)的时间复杂度和O(1)的空间复杂度下将一个降序数组重排为特定模式：先从两端交替取元素再中间向两边取。通过多次反转子数组并交换元素实现目标排列。

Given a descending array A, reorder it to: Ln→L0→Ln-1→L1→Ln-2→L2→…
e.g. A = [7, 6, 5, 4, 3, 2, 1] → A = [1, 7, 2, 6, 3, 5, 4]

NOTICE: do this with only O(1) memory space and in O(n) time complexity.

public void change(int[] arr){
	int start = 0;
	int end = arr.length - 1;
	int seg = arr.length / 3;
	//System.out.println("seg : " + seg);		
	reverse(arr, start, end);
	start += 2;
	reverse(arr, start, end);

	for(int i = 0; i < seg; i++){				
		start += 3;
		reverse(arr, start, end);
	}
	int point = 1;
	for(int i = 0; i < seg; i++){
		swap(arr, point + i * 3, point + i * 3 + 1);
	}		
}

private void reverse(int[] arr, int start, int end){
	while(start < end){
		int temp = arr[start];
		arr[start] = arr[end];
		arr[end] = temp;
		start++;
		end--;
	}
}