[leetcode]264. Ugly Number II

题目链接：https://leetcode.com/problems/ugly-number-ii/#/description

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, and n does not exceed 1690.

We have an array k of first n ugly number. We only know, at the beginning, the first one, which is 1. Then

k[1] = min( k[0]x2, k[0]x3, k[0]x5). The answer is k[0]x2. So we move 2's pointer to 1. Then we test:

k[2] = min( k[1]x2, k[0]x3, k[0]x5). And so on. Be careful about the cases such as 6, in which we need to forward both pointers of 2 and 3.

x here is multiplication.

class Solution {
public:
    int nthUglyNumber(int n) {
        if(n<=0) return false;
        if(n==1) return true;
        int t2=0,t3=0,t5=0;
        vector<int> k(n);
        k[0]=1;
        for(int i=1;i<n;i++)
        {
            k[i]=min(k[t2]*2,min(k[t3]*3,k[t5]*5));
            if(k[i]==k[t2]*2) t2++;
            if(k[i]==k[t3]*3) t3++;
            if(k[i]==k[t5]*5) t5++;
        }
        return k[n-1];
    }
};