card shuffler - Google Interview



card shuffler:shuffle的程序是一个简单的array,array里的值代表当前位置卡片的下一个位置
e.g 当前卡片位置ABCDE shuffle array是01234的话下个卡片位置还是ABCDE,43210的话下个卡片位置是EDCBA。
问给定一个shuffle array,不断用这个array去shuffle,能否得到最初的card deck,能得话要多少次。

我能想到的做法是一个一个的字母找,看多少次能回到原来的位置(或者不能回到原来的位置),记录这个次数为t[i], 然后求所有t[i]的LCM最小公倍数,
求最小公倍数的方法是先求最大公约数(用辗转相除法),然后再用a*b/gcd(a,b) 求最小公倍数。
X.
public int cardShuffler(int[] cards, int[] shuffle) {
int[] steps = new int [cards.length];
for(int i=0; i<cards.length; i++) {
steps[i] = stepsBack(i, shuffle);
if (steps[i] == -1)
return -1;
}
return LCM(steps);
}
private int stepsBack(int start, int[] shuffle) {
int i = start;
int res = 0;
Set visited = new HashSet();
do {
i = shuffle[i];
if (visited.contains(i))
return -1;
visited.add(i);
res++;
} while(i != start);
return res;
}
private int LCM(int[] steps) {
int res = steps[0];
for(int i=1; i<steps.length; i++) {
res = res*steps[i]/GCD(res, steps[i]);
}
return res;
}
private int GCD(int a, int b) {
int n1 = Math.max(a,b);
int n2 = Math.min(a,b);
while(n1%n2 != 0) {
int tmp = n1%n2;
n1 = n2;
n2 = tmp;
}
return n2;
}
http://shibaili.blogspot.com/2015/07/google-interview-questions-4.html
O(n), 对shuffle里面的每一个点都走一遍,最后走回起始点,走过的路径全部mark起来,记录count。mark过的可以直接跳过
最后取所以count的最小公约数即可 
int getSmallest(int i, int j) {
    for (int s = 1; s <= j; s++) {
        if (s * i % j == 0) return s * i;
    }
}
 
int jumpBack(vector<int> &shuffle) {
    vector<int> visit(shuffle.size(),-1);
    int count = 0, preCount = 1;
    for (int i = 0; i < shuffle.size(); i++) {
        count = 0;
        int index = i;
        while (visit[index] == -1) {
            count++;
            visit[index] = shuffle[i];
            index = shuffle[index];
            if (visit[index] != -1 && visit[index] != visit[i]) return -1;
        }
        if (count != 0) preCount = getSmallest(count,preCount);   
                
    }
     
    return preCount;
}

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