Codility - Peaks 문제 풀이 - Henry's Algorithm

 

A non-empty array A consisting of N integers is given.

A peak is an array element which is larger than its neighbors. More precisely, it is an index P such that 0 < P < N − 1,  A[P − 1] < A[P] and A[P] > A[P + 1].

For example, the following array A:

A[0] = 1 A[1] = 2 A[2] = 3 A[3] = 4 A[4] = 3 A[5] = 4 A[6] = 1 A[7] = 2 A[8] = 3 A[9] = 4 A[10] = 6 A[11] = 2

has exactly three peaks: 3, 5, 10.

We want to divide this array into blocks containing the same number of elements. More precisely, we want to choose a number K that will yield the following blocks:

• A[0], A[1], ..., A[K − 1],
• A[K], A[K + 1], ..., A[2K − 1],
  ...
• A[N − K], A[N − K + 1], ..., A[N − 1].

What's more, every block should contain at least one peak. Notice that extreme elements of the blocks (for example A[K − 1] or A[K]) can also be peaks, but only if they have both neighbors (including one in an adjacent blocks).

The goal is to find the maximum number of blocks into which the array A can be divided.

Array A can be divided into blocks as follows:

• one block (1, 2, 3, 4, 3, 4, 1, 2, 3, 4, 6, 2). This block contains three peaks.
• two blocks (1, 2, 3, 4, 3, 4) and (1, 2, 3, 4, 6, 2). Every block has a peak.
• three blocks (1, 2, 3, 4), (3, 4, 1, 2), (3, 4, 6, 2). Every block has a peak. Notice in particular that the first block (1, 2, 3, 4) has a peak at A[3], because A[2] < A[3] > A[4], even though A[4] is in the adjacent block.

However, array A cannot be divided into four blocks, (1, 2, 3), (4, 3, 4), (1, 2, 3) and (4, 6, 2), because the (1, 2, 3) blocks do not contain a peak. Notice in particular that the (4, 3, 4) block contains two peaks: A[3] and A[5].

The maximum number of blocks that array A can be divided into is three.

Write a function:

class Solution { public int solution(int[] A); }

that, given a non-empty array A consisting of N integers, returns the maximum number of blocks into which A can be divided.

If A cannot be divided into some number of blocks, the function should return 0.

For example, given:

A[0] = 1 A[1] = 2 A[2] = 3 A[3] = 4 A[4] = 3 A[5] = 4 A[6] = 1 A[7] = 2 A[8] = 3 A[9] = 4 A[10] = 6 A[11] = 2

the function should return 3, as explained above.

Write an efficient algorithm for the following assumptions:

• N is an integer within the range [1..100,000];
• each element of array A is an integer within the range [0..1,000,000,000].

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Peaks 문제는 봉우리를 최소 1개씩 포함하고 있는 Block의 개수를 구하는 문제이다.

 

최대 블록의 개수를 Return 해주면 되는데,

그 방법을 구체적으로 어떻게 짜야되는지 다양하게 시도하다가 

결국 답지를 보고 풀었다.ㅜㅜ!!

 

그래도 이 방법 잘 알아두어야겠다.

 

정답 코드

# you can write to stdout for debugging purposes, e.g.
# print("this is a debug message")

def solution(A):
    # write your code in Python 3.6
    
    peaks = []
    
    for i in range(1, len(A)-1):
        if A[i] > A[i-1] and A[i] > A[i+1]:
            peaks.append(i)
            
    for i in range(len(peaks),0,-1):
        if len(A) % i == 0:
            block_size = len(A)//i
            block = [False]*i
            block_cnt = 0
            for j in range(len(peaks)):
                idx = peaks[j] // block_size
                if block[idx] == False:
                    block[idx] = True
                    block_cnt += 1
            
            if block_cnt == i:
                return block_cnt
        
    return 0
        
    pass