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66 lines
1.9 KiB
Python
66 lines
1.9 KiB
Python
"""
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Kadane's algorithm to get maximum subarray sum
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https://medium.com/@rsinghal757/kadanes-algorithm-dynamic-programming-how-and-why-does-it-work-3fd8849ed73d
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https://en.wikipedia.org/wiki/Maximum_subarray_problem
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"""
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test_data: tuple = ([-2, -8, -9], [2, 8, 9], [-1, 0, 1], [0, 0], [])
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def negative_exist(arr: list) -> int:
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"""
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>>> negative_exist([-2,-8,-9])
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-2
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>>> [negative_exist(arr) for arr in test_data]
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[-2, 0, 0, 0, 0]
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"""
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arr = arr or [0]
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max = arr[0]
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for i in arr:
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if i >= 0:
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return 0
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elif max <= i:
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max = i
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return max
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def kadanes(arr: list) -> int:
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"""
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If negative_exist() returns 0 than this function will execute
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else it will return the value return by negative_exist function
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For example: arr = [2, 3, -9, 8, -2]
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Initially we set value of max_sum to 0 and max_till_element to 0 than when
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max_sum is less than max_till particular element it will assign that value to
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max_sum and when value of max_till_sum is less than 0 it will assign 0 to i
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and after that whole process, return the max_sum
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So the output for above arr is 8
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>>> kadanes([2, 3, -9, 8, -2])
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8
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>>> [kadanes(arr) for arr in test_data]
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[-2, 19, 1, 0, 0]
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"""
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max_sum = negative_exist(arr)
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if max_sum < 0:
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return max_sum
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max_sum = 0
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max_till_element = 0
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for i in arr:
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max_till_element += i
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if max_sum <= max_till_element:
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max_sum = max_till_element
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if max_till_element < 0:
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max_till_element = 0
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return max_sum
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if __name__ == "__main__":
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try:
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print("Enter integer values sepatated by spaces")
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arr = [int(x) for x in input().split()]
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print(f"Maximum subarray sum of {arr} is {kadanes(arr)}")
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except ValueError:
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print("Please enter integer values.")
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