# Given an array, find the number of all pairs with odd sum.

**Objective: **Given an array of integers, write a program to find the number of pairs with even odd.

**Example:**

Given Input: [1, 2, 3, 4] Number of odd pairs: 4 Note: (1, 2), (1, 4), (2, 3) and (3, 4) Given Input: [6, 7, 1, 3, 2, 5, 4] Number of odd pairs

**Naive approach: **Use nested loops and check the sum of each possible pair and if the sum is odd then add it to the result.

Time Complexity: O(N^{2})

**Better approach:**

- Iterate through the array and count the number of odd elements and even elements.
- We know the
, we will use this property.*odd + even = odd* - Number of pairs with odd count = evenCount*oddCount.

Time Complexity: O(N)

**Complete Code:**

**Output:**

Given Input: [6, 7, 1, 3, 2, 5, 4] Number of odd pairs: 12