294. Flip Game II
题目:
You are playing the following Flip Game with your friend: Given a string that contains only these two characters: +
and -
, you and your friend take turns to flip twoconsecutive "++"
into "--"
. The game ends when a person can no longer make a move and therefore the other person will be the winner.
Write a function to determine if the starting player can guarantee a win.
For example, given s = "++++"
, return true. The starting player can guarantee a win by flipping the middle "++"
to become "+--+"
.
Follow up:
Derive your algorithm's runtime complexity.
链接: http://leetcode.com/problems/flip-game-ii/
题解:
求是否startng player可以有一种策略保证赢取游戏。直觉就是dfs +backtracking。 代码和Flip Game I基本一样,不过加入了验证下一步的一个条件语句。假如下一步next,对手不能赢,则这一步我们可以赢。看了Discuss以后发现还可以有O(n2)的DP做法,有些Game Theory的成分。Stellari大神好厉害。
Time Complexity - O(2n), Space Complexity - O(2n)
public class Solution {
public boolean canWin(String s) {
char[] arr = s.toCharArray();
for(int i = 1; i < s.length(); i++) {
if(arr[i] == '+' && arr[i - 1] == '+') {
arr[i] = '-';
arr[i - 1] = '-';
String next = String.valueOf(arr);
if(!canWin(next)) {
return true;
}
arr[i] = '+';
arr[i - 1] = '+';
}
} return false;
}
}
Reference:
https://leetcode.com/discuss/64344/theory-matters-from-backtracking-128ms-to-dp-0ms
https://leetcode.com/discuss/64291/share-my-java-backtracking-solution
https://leetcode.com/discuss/64522/simple-backtracking-inspired-by-flip-game-i
https://leetcode.com/discuss/64357/memoization-3150ms-130ms-44ms-python
https://leetcode.com/discuss/64486/backtracking-solution-time-optimization-through-205ms-19ms
https://leetcode.com/discuss/64350/short-java-%26-ruby
https://leetcode.com/discuss/64293/1-line-python-solution
https://leetcode.com/discuss/64332/java-recursive-backtracking-solution-27ms
https://leetcode.com/discuss/64302/easy-to-understand-java-solution
http://lucida.me/blog/developer-reading-list/
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