# Greedy Algorithms

Greedy algorithms make the decision of what to do next by selecting the best local option from all available choices without regard to the global structure. This heuristic produces the optimum solution for some problems.

## 3Sum

Given an array and a number, determine if there are three entries in the array which add up to the specified number.

The brute force algorithm is to consider all possible triples ($$O(n^3)$$). We can improve the time complexity to $$O(n^2)$$ by using a linear version of 2Sum. The 2Sum implementation below first places all array items in a set. Note that we could return the indices by storing items in a dictionary with item keys and index values.

def has_three_sum(array, target):
return any(has_two_sum(array, target - item) for item in array)

def has_two_sum(array, target):
values = set(array)
return any(target - item in values for item in array)


We can remove the need for $$O(n)$$ space with a greedy algorithm. First, sort the given array. Then, compare $$A[0] + A[n - 1]$$ with the target. If the sum is smaller than the target, move to $$A[1] + A[n - 1]$$. If the sum is larger than the target, move to $$A[0] + A[n - 2]$$. Follow this logic until either the target is reached or the indices meet.

def has_three_sum(array, target):
array.sort()
return any(has_two_sum(array, target - item) for item in array)

def has_two_sum(array, target):
left, right = 0, len(array) - 1
while left < right:
total = array[left] + array[right]
if total == target:
return True
elif total < target:
left += 1
else:
right -= 1
return False