Leetcode 264. Ugly Number II

264. Ugly Number II

• Total Accepted: 35338
• Total Submissions: 120258
• Difficulty: Medium

Write a program to find the n-th ugly number.

Ugly numbers are positive numbers whose prime factors only include 2, 3, 5. For example, 1, 2, 3, 4, 5, 6, 8, 9, 10, 12 is the sequence of the first 10 ugly numbers.

Note that 1 is typically treated as an ugly number.

思路：动态规划。维持3条线路，uglynum_new=min(uglynum[k2]*2,uglynum[k3]*3,uglynum[k5]*3)(k2,k3,k5从0开始，uglynum[0]=1)。但要注意，可能会出现uglynum_new=uglynum[k2]*2=uglynum[k3]*3=uglynum[k5]*3的情况，此时，k2，k3，k5都要加1。

代码：

class Solution {
public:
    int minnum(int a,int b,int c){
        return c<(min(a,b))?c:min(a,b);
    }
    int nthUglyNumber(int n) {
        vector<int> uglynum(1,1);
        int k2=0,k3=0,k5=0;
        while(uglynum.size()<n){
            int temp=minnum(uglynum[k2]*2,uglynum[k3]*3,uglynum[k5]*5);
            uglynum.push_back(temp);
            if(temp==uglynum[k2]*2){
                k2++;
            }
            if(temp==uglynum[k3]*3){
                k3++;
            }
            if(temp==uglynum[k5]*5){
                k5++;
            }
        }
        return uglynum[n-1];
    }
};