LeetCode 2444. Count Subarrays With Fixed Bounds

原题链接在这里：https://leetcode.com/problems/count-subarrays-with-fixed-bounds/

题目：

You are given an integer array nums and two integers minK and maxK.

A fixed-bound subarray of nums is a subarray that satisfies the following conditions:

• The minimum value in the subarray is equal to minK.
• The maximum value in the subarray is equal to maxK.

Return the number of fixed-bound subarrays.

A subarray is a contiguous part of an array.

Example 1:

Input: nums = [1,3,5,2,7,5], minK = 1, maxK = 5
Output: 2
Explanation: The fixed-bound subarrays are [1,3,5] and [1,3,5,2].

Example 2:

Input: nums = [1,1,1,1], minK = 1, maxK = 1
Output: 10
Explanation: Every subarray of nums is a fixed-bound subarray. There are 10 possible subarrays.

Constraints:

• 2 <= nums.length <= 105
• 1 <= nums[i], minK, maxK <= 106

题解：

For valid subarray, it must have both minK and maxK.

When iterate nums[i], it is out of [minK, maxK], we need to set the start of subarray as i + 1.

When we see minK and maxK, update last seen index for minK and maxK.

The start of subarray could be choosen from j to min(lastMinInd, lastMaxInd). 

e.g. [1, 3, 5, 1, 7, 5]. minK = 1, maxK = 5.

When ind = 3, num = 1. start of subarray could be 1, 3, 5. The corresponding subarray is [1, 3, 5, 1], [3, 5, 1] and [5, 1].

Time Complexity: O(n). n = nums.length.

Space: O(1).

AC Java:

class Solution {
    public long countSubarrays(int[] nums, int minK, int maxK) {
        int n = nums.length;
        int startInd = 0;
        int lastMinInd = -1;
        int lastMaxInd = -1;
        long res = 0;
        for(int i = 0; i < n; i++){
            if(nums[i] < minK || nums[i] > maxK){
                lastMinInd = lastMaxInd = -1;
                startInd = i + 1;
            }
            
            if(nums[i] == minK){
                lastMinInd = i;
            }
            
            if(nums[i] == maxK){
                lastMaxInd = i;
            }
            
            res += Math.max(0L, Math.min(lastMinInd, lastMaxInd) - startInd + 1);
        }
        
        return res;
    }
}

类似Number of Subarrays with Bounded Maximum, Count Number of Nice Subarrays.