464. Can I Win

Posted 2021-01-10 bernieloveslife

In the "100 game," two players take turns adding, to a running total, any integer from 1..10. The player who first causes the running total to reach or exceed 100 wins.

What if we change the game so that players cannot re-use integers?

For example, two players might take turns drawing from a common pool of numbers of 1..15 without replacement until they reach a total >= 100.

Given an integer maxChoosableInteger and another integer desiredTotal, determine if the first player to move can force a win, assuming both players play optimally.

You can always assume that maxChoosableInteger will not be larger than 20 and desiredTotal will not be larger than 300.

Example

Input:
maxChoosableInteger = 10
desiredTotal = 11

Output:
false

Explanation:
No matter which integer the first player choose, the first player will lose.
The first player can choose an integer from 1 up to 10.
If the first player choose 1, the second player can only choose integers from 2 up to 10.
The second player will win by choosing 10 and get a total = 11, which is >= desiredTotal.
Same with other integers chosen by the first player, the second player will always win.

class Solution(object):
    def canIWin(self, maxChoosableInteger, desiredTotal):
        """
        :type maxChoosableInteger: int
        :type desiredTotal: int
        :rtype: bool
        """

        total = maxChoosableInteger * (maxChoosableInteger + 1) // 2
        if total < desiredTotal:
            return False
        if total == desiredTotal:
            return (maxChoosableInteger % 2 == 1)
        if maxChoosableInteger >= desiredTotal:
            return True
        mask = 1 << maxChoosableInteger
        self.record = {}
        return self.checkWin(maxChoosableInteger, mask, desiredTotal)

    def checkWin(self, target, mask, remain):
        if mask in self.record:
            return self.record[mask]
        for i in range(target):
            if (1 & (mask >> i)) != 0:
                continue
            n = i + 1  # n: picked number
            if (n >= remain) or (self.checkWin(target, mask | (1 << i), remain - n) is False):
                self.record[mask] = True
                return True
        self.record[mask] = False
        return False

好难。
首先要注意使用一个二进制数来保存状态空间，如果用字符串会MLE。这意味着会有一些让人头疼的位运算。
这样可以给字典的键赋一个初始值 d = collections.defaultdict(lambda : False)