Ugly Number II

记录一下用Index代替list的思想，非常简洁，节省空间

class Solution {
public:
	//求三个数的最小值
	int min(int a, int b, int c) {
		int minNum = a > b ? b : a;
		return minNum > c ? c : minNum;
	}
	//求第N个丑数
	int nthUglyNumber(int n) {
		vector<int> ugly(n,0);
		ugly[0] = 1;

		int factor2 = 2, factor3 = 3, factor5 = 5;
		int index2, index3, index5;
		index2 = index3 = index5 = 0;

		for (int i = 1; i < n; i++) {
			int minNum = min(factor2, factor3, factor5);
			ugly[i] = minNum;

			if (factor2 == minNum)
				factor2 = 2 * ugly[++index2];
			if (factor3 == minNum)
				factor3 = 3 * ugly[++index3];
			if (factor5 == minNum)
				factor5 = 5 * ugly[++index5];
		}

		return ugly[n - 1];
	}
};