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Prefix Sum Array – Implementation and Applications in Competitive Programming

Given an array arr[] of size N, find the prefix sum of the array. A prefix sum array is another array prefixSum[] of the same size, such that the value of prefixSum[i] is arr[0] + arr[1] + arr[2] . . . arr[i].

Examples:
Input: arr[] = {10, 20, 10, 5, 15}
Output: prefixSum[] = {10, 30, 40, 45, 60}
Explanation: While traversing the array, update the element by adding it with its previous element.
prefixSum[0] = 10,
prefixSum[1] = prefixSum[0] + arr[1] = 30,
prefixSum[2] = prefixSum[1] + arr[2] = 40 and so on.

write your answer in the group or send it with the bot i will review it.

Answer:

def prefix_sum(arr):
    prefix_sum_arr = [0] * len(arr)
    prefix_sum_arr[0] = arr[0]
    for i in range(1, len(arr)):
        prefix_sum_arr[i] = prefix_sum_arr[i - 1] + arr[i]
    return prefix_sum_arr

arr = [10, 20, 10, 5, 15]
result = prefix_sum(arr)
print(result)  # Output: [10, 30, 40, 45, 60]

#leet_codeQ16 #Q_209 #Easy #Prefix_sum
1991. Find the Middle Index in Array

Hint
Given a 0-indexed integer array nums, find the leftmost middleIndex (i.e., the smallest amongst all the possible ones).

A middleIndex is an index where nums[0] + nums[1] + ... + nums[middleIndex-1] == nums[middleIndex+1] + nums[middleIndex+2] + ... + nums[nums.length-1].

If middleIndex == 0, the left side sum is considered to be 0. Similarly, if middleIndex == nums.length - 1, the right side sum is considered to be 0.

Return the leftmost middleIndex that satisfies the condition, or -1 if there is no such index.



Example 1:

Input: nums = [2,3,-1,8,4]
Output: 3
Explanation: The sum of the numbers before index 3 is: 2 + 3 + -1 = 4
The sum of the numbers after index 3 is: 4 = 4
Example 2:

Input: nums = [1,-1,4]
Output: 2
Explanation: The sum of the numbers before index 2 is: 1 + -1 = 0
The sum of the numbers after index 2 is: 0
Example 3:

Input: nums = [2,5]
Output: -1
Explanation: There is no valid middleIndex.


Constraints:

1 <= nums.length <= 100
-1000 <= nums[i] <= 1000

Mean of range in array
#Q17 Geeks for Geeks
Given an array of n integers and q queries. Write a program to find floor value of mean in range l to r for each query in a new line.
Queries are given by an array queries[] of size 2*q. Here queries[2*i] denote l and queries[2*i+1] denote r for i-th query (0<= i <q).

Example 1:

Input : Arr[] = {1, 2, 3, 4, 5}, Q = 3
queries[] = {0, 2, 1, 3, 0, 4}
Output : 2 3 3
Explanation:
Here we can see that the array of
integers is [1, 2, 3, 4, 5].
Query 1: L = 0 and R = 2
Sum = 6
Integer Count = 3
So, Mean is 2
Query 2: L = 1 and R = 3
Sum = 9
Integer Count = 3
So, Mean is 3
Query 3: L = 0 and R = 4
Sum = 15
Integer Count = 5
So, the Mean is 3.
So, In the end, the function will
return the array [2, 3, 3] as an answer.

Example 2:

Input : Arr[] = {6, 7, 8, 10}, Q = 2
queries[] = {0, 3, 1, 2}
Output : 7 7

Try to Solve this problem

#Q18 #leet_codeQ18 Easy
1480 . Given an array nums. We define a running sum of an array as runningSum[i] = sum(nums[0]…nums[i]).

Return the running sum of nums.


Example 1:

Input: nums = [1,2,3,4]
Output: [1,3,6,10]
Explanation: Running sum is obtained as follows: [1, 1+2, 1+2+3, 1+2+3+4].
Example 2:

Input: nums = [1,1,1,1,1]
Output: [1,2,3,4,5]
Explanation: Running sum is obtained as follows: [1, 1+1, 1+1+1, 1+1+1+1, 1+1+1+1+1].
Example 3:

Input: nums = [3,1,2,10,1]
Output: [3,4,6,16,17]


Constraints:

1 <= nums.length <= 1000
-10^6 <= nums[i] <= 10^6

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