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);
     }
 
 }