Shuffling algorithm Fisher-Yates shuffle

Returns a random number between 1-N in the int rand (int n) function. How to disrupt array a [n?

The original Fisher-Yates algorithm opened another array B [N] and put the expected a [n] in B [N]. The steps are as follows:

Set I = 0

1. Generate a random number k between 1-a.length

2. Place a [k] to B [I] to remove a [k] And I ++ from array.

3. If a. length> 0, go to step 1.

This algorithm needs to remove the number in a and shift each time, so the algorithm complexity is O (n ^ 2 ).

For example, an algorithm process is as follows:

Random Number range	Random Number	A	B
		1 2 3 4 5 6 7 8

Random Number range	Random Number	A	B
1-8	3	1 234 5 6 7 8	3

Random Number range	Random Number	A	B
1-7	4	1 23456 7 8	3 5

Random Number range	Random Number	A	B
1-6	5	1 2345678	3 5 7
1-5	3	1 23 4 5678	3 5 7 4
1-4	4	1 23 4 567 8	3 5 7 4 8
1-3	1	123 4 567 8	3 5 7 4 8 1
1-2	2	123 4 5 6 7 8	3 5 7 4 8 1 6
		1 2 3 4 5 6 7 8	3 5 7 4 8 1 6 2

Later, the algorithm was improved. Instead of opening an array, it switched the elements above array a to achieve the shuffling.

This algorithm has two versions, the same principle:

Version 1:

For (INT I = N; I> = 1; -- I) {Int J = rand (I); // generates a random number exchange (A [I], A [J]); // exchange a [I], a [J]}

Version 2:

For (INT I = 1; I <= N; + I) {Int J = (RAND (N)/n) * (N-I + 1) + I-1; // generate a random number exchange (A [I], a [J]) between I-n; // exchange a [I], a [J]

A calculation process of version 1 is as follows:

Random Number range random number AA (sorted) 1-861 2 3 4 5 87 6

Random Number range	Random Number	A	A (sorted)
1-7	2	173 4 5 8	26

Random Number range	Random Number	A	A (sorted)
1-6	6	1 7 3 4 5	82 6
1-5	1	57 3 4	18 2 6
1-4	3	5 74	31 8 2 6
1-3	3	5 7	43 1 8 2 6
1-2	1	7	54 3 1 8 2 6

Reference: http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle

Shuffling algorithm Fisher-Yates shuffle