【剑指offer-Java版】34丑数

丑数:返回第N个丑数

只包含因子 2 3 5的数称为丑数，第一个丑数是 1

采用辅助数组的方法，提高时间效率 – 下一个丑数一定是已有的丑数乘以2 或者 3 或者 5 得到的

    public class _Q34<T> {


    public int GetUglyNumber(int count){

        if(count < 1) return 0;
        if(count == 1) return 1;

        int uglyArray[] = new int[count];
        uglyArray[0] = 1; // 第一个丑数是 1

        int nextUglyIndex = 1;
        int nextUglyNum = 0;

        int index2 = 0; // 记录第一个满足uglyArray[]*2 > 当前最大丑数 的下标
        int index3 = 0;
        int index5 = 0;

        while(nextUglyIndex < count){
            nextUglyNum = MinInThree((uglyArray[index2]*2), (uglyArray[index3]*3), (uglyArray[index5]*5));

            uglyArray[nextUglyIndex] = nextUglyNum;

            while(uglyArray[index2]*2 <= uglyArray[nextUglyIndex]){
                index2++;
            }

            while(uglyArray[index3]*3 <= uglyArray[nextUglyIndex]){
                index3++;
            }

            while(uglyArray[index5]*5 <= uglyArray[nextUglyIndex]){
                index5++;
            }

            nextUglyIndex++;
        }


        return uglyArray[nextUglyIndex-1];
    }

    private int MinInThree(int num1, int num2, int num3){
        //return (num1 < num2 ? num1:num2) < num3 ? (num1 < num2 ? num1:num2) : num3;
        int min = num1 < num2 ? num1 : num2;
        min = min < num3 ? min : num3;
        return min;
    }
    }

测试代码：

    public class _Q34Test extends TestCase {

    _Q34 ugly = new _Q34();

    public void test(){
        System.out.println(ugly.GetUglyNumber(1));
        System.out.println(ugly.GetUglyNumber(2));
        System.out.println(ugly.GetUglyNumber(3));
        System.out.println(ugly.GetUglyNumber(4));
        System.out.println(ugly.GetUglyNumber(5));
        System.out.println(ugly.GetUglyNumber(6));
        System.out.println(ugly.GetUglyNumber(7));
        System.out.println(ugly.GetUglyNumber(8));
        System.out.println(ugly.GetUglyNumber(9));
        System.out.println(ugly.GetUglyNumber(10));
        System.out.println(ugly.GetUglyNumber(11));

        System.out.println(ugly.GetUglyNumber(0));
        System.out.println(ugly.GetUglyNumber(1500));
    }
    }