LWC 72: 786. K-th Smallest Prime Fraction
2018-02-18 18:46
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LWC 72: 786. K-th Smallest Prime Fraction
传送门:786. K-th Smallest Prime FractionProblem:
A sorted list A contains 1, plus some number of primes. Then, for every p < q in the list, we consider the fraction p/q.
What is the K-th smallest fraction considered? Return your answer as an array of ints, where answer[0] = p and answer1 = q.
Examples:
Input: A = [1, 2, 3, 5], K = 3
Output: [2, 5]
Explanation:
The fractions to be considered in sorted order are:
1/5, 1/3, 2/5, 1/2, 3/5, 2/3.
The third fraction is 2/5.
Input: A = [1, 7], K = 1
Output: [1, 7]
Note:
A will have length between 2 and 2000.
Each A[i] will be between 1 and 30000.
K will be between 1 and A.length * (A.length + 1) / 2.
思路1:
一种聪明的做法,如果A = [1, 7, 23, 29, 47],那么有:
1/47 < 1/29 < 1/23 < 1/7 7/47 < 7/29 < 7/23 23/47 < 23/29 29/47
可以采用优先队列的方法,把第一列先送入队列,因为他们一定是每行的最小,选出最小的之后,加入该行的后续元素。直到找到K-1个最小的元素。
代码如下:
public int[] kthSmallestPrimeFraction(int[] a, int k) { int n = a.length; PriorityQueue<int[]> pq = new PriorityQueue<>(new Comparator<int[]>() { @Override public int compare(int[] o1, int[] o2) { int s1 = a[o1[0]] * a[o2[1]]; int s2 = a[o2[0]] * a[o1[1]]; return s1 - s2; } }); for (int i = 0; i < n-1; i++) { pq.add(new int[]{i, n-1}); } for (int i = 0; i < k-1; i++) { int[] pop = pq.poll(); if (pop[1] - 1 > pop[0]) { pop[1]--; pq.offer(pop); } } int[] peek = pq.peek(); return new int[]{a[peek[0]], a[peek[1]]}; }
思路2:
二分,先找到这个第k小的数,接着再小范围暴力搜索,居然能过。
代码如下:
public int[] kthSmallestPrimeFraction(int[] a, int K) { double low = 0, high = 1; double eps = 1e-8; int n = a.length; for(int rep = 0; rep < 50; ++rep){ double x = low + (high-low) / 2; int num = 0; for(int i = 0;i < n;i++){ int ind = Arrays.binarySearch(a, (int)(x * a[i])); if(ind < 0) ind = -ind - 2; num += ind + 1; } if(num >= K){ high = x; }else{ low = x; } } for (int i = 0; i < n; ++i) { double find = a[i] * high; int idx = Arrays.binarySearch(a, (int)find); if (idx < 0) idx = -idx; for (int j = -1; j <= 1; ++j) { if (j + idx >= 0 && j + idx < n && Math.abs(a[j + idx] - find) < eps) { return new int[] {a[j + idx], a[i]}; } } } return new int[]{-1, -1}; }
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