2020-08-22

679. 24 点游戏

DFS暴力，每次在数组中找两个数进行组合，一直到剩下一个数为止。

class Solution(object):
    def judgePoint24(self, nums):
        """
        :type nums: List[int]
        :rtype: bool
        """
        def dfs(nums):
            if len(nums)==1:
                if abs(nums[0]-24) < 1e-6:
                    #print(ans)
                    return True
                return False

            for i in range(len(nums)):
                for j in range(len(nums)):
                    if i==j: continue
                    newnum1 = nums[i]+nums[j]
                    newnum2 = nums[i]-nums[j]
                    newnum3 = nums[i]*nums[j]
                    
                    remain = [nums[k] for k in range(len(nums)) if(k!=i and k!=j)]
                    #ans.append(['(',nums[i],'+',nums[j],')'])
                    if dfs(remain+[newnum1]): return True
                    #ans.pop()
                    #ans.append(['(',nums[i],'-',nums[j],')'])
                    if dfs(remain+[newnum2]): return True
                    #ans.pop()
                    #ans.append(['(',nums[i],'*',nums[j],')'])
                    if dfs(remain+[newnum3]): return True
                    #ans.pop()
                    if nums[j]==0: continue
                    #ans.append(['(',nums[i],'/',nums[j],')'])
                    newnum4 = nums[i]*1.0/nums[j]
                    if dfs(remain+[newnum4]): return True
                    #ans.pop()
            return False

        return dfs(nums)

5498. 石子游戏 V

题解: 动态规划， dp[i][j]代表 划分区间(i,j) 能够获得的最大价值，分情况讨论即可。

class Solution(object):
    def stoneGameV(self, stoneValue):
        """
        :type stoneValue: List[int]
        :rtype: int
        """
        n = len(stoneValue)
        
        d = {}
        def dfs(start, end):
            if start>=end: return 0
            if (start,end) in d: return d[(start,end)]
            S,now = 0,0
            for i in range(start, end+1):
                S+=stoneValue[i]
            
            ans = 0
            first = 0
            for i in range(start, end):
                first += stoneValue[i]
                second = S - first
                if first<second:
                    ans = max(ans, dfs(start,i)+first) # 前一段比后一段小
                elif first>second:
                    ans = max(ans, dfs(i+1,end)+second) # 后一段比前一段小
                else:
                    ans = max(ans, max(dfs(start,i),dfs(i+1,end))+first) # 两端一样大
            
            d[(start,end)] = ans
            return ans
            
        return dfs(0, n-1)