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Leetcode Week14 Ugly Number II

Question

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

Ugly numbers are positive numbers whose prime factors only include 2, 3, 5

Example:

Input: n = 10
Output: 12
Explanation: 1, 2, 3, 4, 5, 6, 8, 9, 10, 12 is the sequence of the first 10 ugly numbers.

Note:  

  1. 1 is typically treated as an ugly number.
  2. n does not exceed 1690.

Answer

class Solution {
public:
    int nthUglyNumber(int n) {
        if (n <= 0) return 0;
        vector<int> vec(1, 1);
        int p = 0, q = 0, z = 0;
        int minVal = 1;
        for (int i = 1; i < n; i++) {
            minVal 
= min(min(vec[p] * 2, vec[q] * 3), vec[z] * 5); vec.push_back(minVal); if (vec[p] * 2 == minVal) p++; if (vec[q] * 3 == minVal) q++; if (vec[z] * 5 == minVal) z++; } return minVal; } };

  既然是隻有2,3,5的因子,只需將他們每種情況都考慮了就可以了,排序相當於在min的功能做了。