與一般DP不同的是 青蛙能跳k-1，k，k+1步三種不同的情況 所以用字典儲存跳到這個石頭上所用的步數

class Solution(object):
    def canCross(self, stones):
        """
        :type stones: List[int]
        :rtype: bool
        """
        #maxJumpstep 記錄的是上一次跳過來的步數
        if len(stones)<=1: return True
        elif stones[1]!=1: return False 
        else:
            maxJumpstep={s:set() for s in stones}
            maxJumpstep[1].add(1)
            for s in stones[1:-1]:
                for j in maxJumpstep[s]:
                        for jump in (j-1,j,j+1):
                            if jump>0 and s+jump in maxJumpstep:
                                maxJumpstep[s+jump].add(jump)
            return len(maxJumpstep[stones[-1]])>0