LWC 74: 795. Number of Subarrays with Bounded Maximum
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LWC 74: 795. Number of Subarrays with Bounded Maximum
传送门:795. Number of Subarrays with Bounded Maximum
Problem:
We are given an array A of positive integers, and two positive integers L and R (L <= R).
Return the number of (contiguous, non-empty) subarrays such that the value of the maximum array element in that subarray is at least L and at most R.
Example :
Input:
A = [2, 1, 4, 3]
L = 2
R = 3
Output: 3
Explanation: There are three subarrays that meet the requirements: [2], [2, 1], [3].
Note:
- L, R and A[i] will be an integer in the range [0, 10^9].
- The length of A will be in the range of [1, 50000].
思路:
先关注一波性质,在L和R之间的最大值val符合 val in [L, R], 可以转为:求区间内的任意值x , x in [L, R]。所以,把数组的每个位置当作起点,如A = [2, 1, 4, 3]:
位置0: [2], [2, 1], [2, 1, 4], [2, 1, 4, 3]
位置1: [1], [1, 4], [1, 4, 3]
位置2: [4], [4, 3]
位置3: [3]
for each subset in each position:
calculate whether the last value in subset <= R
你会发现,考虑位置0,subset[2, 1, 4]中的last value 4时,因为4 > R,所以删除该subset,且连同位置1,2,3中包含4的子集可以一并删除。
代码如下:
public int numSubarrayBoundedMax(int[] A, int L, int R)
return count(A, R) -count(A, L - 1);
int count(int[] a, int r)
int ret = 0;
int cnt = 0;
for (int v : a)
if (v <= r) cnt ++;
else cnt = 0;
ret += cnt;
return ret;
Python版本:
class Solution(object):
def numSubarrayBoundedMax(self, A, L, R):
"""
:type A: List[int]
:type L: int
:type R: int
:rtype: int
"""
def count(A, R):
ret = cnt = 0
for v in A:
cnt = cnt + 1 if v <= R else 0
ret += cnt
return ret
return count(A, R) - count(A, L - 1)
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