Groupon面经Prepare: Max Cycle Length

题目是遇到偶数/2，遇到奇数 *3 + 1的题目，然后找一个range内所有数字的max cycle length。对于一个数字，比如说44，按照题目的公式不停计算，过程是 44, 22, 11, 8, 9 ,1（瞎起的），从44到1的这个sequence的长度，叫做cycle length。然后题目是给一个range，比如[2,300]，求这里面所有数字的cycle length的最大值。follow up跑1到1 million

 一个数一个数慢慢算，如下算法求出每个数的cycle length

package Sorting;
import java.util.*;

public class Solution2 {
    public List<Integer> maxCycleLen(int min, int max) {
        List<Integer> maxCycle = new ArrayList<Integer>();
        for (int i=min; i<=max; i++) {
            helper(maxCycle, i);
        }
        return maxCycle;
    }
    
    public void helper(List<Integer> maxCycle, int num) {
        int len = 1;
        while (num != 1) {
            if (num%2 == 1) num = num*3+1;
            else num = num/2;
            len++;
        }
        maxCycle.add(len);
    }

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Solution2 sol = new Solution2();
        List<Integer> res = sol.maxCycleLen(1, 100000);
        System.out.println(res);
    }

}

follow up: 建立一个Lookup table, 算过的数就不算了

package Sorting;
import java.util.*;

public class Solution3 {
    HashMap<Integer, Integer> map;
    public List<Integer> maxCycleLen(int min, int max) {
        List<Integer> maxCycle = new ArrayList<Integer>();
        map = new HashMap<Integer, Integer>();
        for (int i=min; i<=max; i++) {
            helper(maxCycle, i);
        }
        return maxCycle;
    }
    
    public void helper(List<Integer> maxCycle, int num) {
        int len = 0;
        int numcopy = num;
        while (!map.containsKey(num)) {
            if (num == 1) {
                map.put(1, 1);
                maxCycle.add(1);
                return;
            }
            if (num%2 == 1) num = num*3+1;
            else num = num/2;
            len++;
        }
        len = len + map.get(num);
        maxCycle.add(len);
        map.put(numcopy, len);
    }

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Solution3 sol = new Solution3();
        List<Integer> res = sol.maxCycleLen(1, 100000);
        System.out.println(res);
    }

}