洗牌算法

http://en.wikipedia.org/wiki/Fisher–Yates_shuffle

 An implementation of Durstenfeld's algorithm in Java is:

public static void shuffle(int[] array) {
    Random rng = new Random();  // java.util.Random
    // n is the number of items remaining to be shuffled.
    for (int n = array.length; n > 1; n--) {
        // Pick a random element to swap with the nth element.
        int k = rng.nextInt(n);  // 0 <= k <= n-1 (0-based array)
        // Swap array elements.
        int tmp = array[k];
        array[k] = array[n-1];
        array[n-1] = tmp;
    }
}