LeetCode: Number of Valid Subarrays

Number of Valid Subarrays

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• Tag: #monotone

Given an array A of integers, return the number of non-empty continuous subarrays that satisfy the following condition:

The leftmost element of the subarray is not larger than other elements in the subarray.

Example 1:

Input: [1,4,2,5,3]
Output: 11
Explanation: There are 11 valid subarrays: [1],[4],[2],[5],[3],[1,4],[2,5],[1,4,2],[2,5,3],[1,4,2,5],[1,4,2,5,3].

Example 2:

Input: [3,2,1]
Output: 3
Explanation: The 3 valid subarrays are: [3],[2],[1].

Example 3:

Input: [2,2,2]
Output: 6
Explanation: There are 6 valid subarrays: [2],[2],[2],[2,2],[2,2],[2,2,2].

Note:

1. 1 <= A.length <= 50000
2. 0 <= A[i] <= 100000

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

• Solution:

// https://code.dennyzhang.com/number-of-valid-subarrays
// Basic Ideas: Monotone stack
//    Find the next bigger value for a given element
// Complexity: Time O(n), Space O(n)
func validSubarrays(nums []int) int {
    l := make([]int, len(nums))
    for i, _ := range l {
        l[i] = len(nums)
    }
    stack := []int{}
    for i, v := range nums {
        for len(stack) > 0 && nums[stack[len(stack)-1]] > v {
            l[stack[len(stack)-1]] = i
            stack = stack[0:len(stack)-1]
        }
        stack = append(stack, i)
    }

    res := 0
    for i, _ := range nums {
        res += l[i]-i
    }
    return res
}