Magnetic Force Between Two Balls LeetCode solution in Python

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class Solution:
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    def possible(self, distance, positions, M):
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        ball = 1
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        lastPos = positions[0]
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        for pos in positions:
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            if pos - lastPos >= distance:
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                ball += 1
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                if ball == M:
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                    return True
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                lastPos = pos
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        return False
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    def maxDistance(self, positions, M):
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        positions.sort()
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        low = 0
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        high = positions[-1]
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        ans = 0
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        while low <= high:
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            distance = low + (high - low) // 2
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            if self.possible(distance, positions, M):
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                ans = distance
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                low = distance + 1
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            else:
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                high = distance - 1
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        return ans