Flip Game I && II

Flip Game I

Problem Description:

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 two consecutive

"++"

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 compute all possible states of the string after one valid move.

For example, given

s = "++++"

, after one move, it may become one of the following states:

[
"--++",
"+--+",
"++--"
]

If there is no valid move, return an empty list

[]

.

public List<String> generatePossibleNextMoves(String s) {
List<String> list = new ArrayList<String>();

if (s == null || s.length() < 2) return list;

for (int i = -1; (i = s.indexOf("++", i + 1)) >= 0;) {
list.add(s.substring(0, i) + "--" + s.substring(i + 2));
}
return list;
}

Flip Game II

Problem Description:

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 two consecutive

"++"

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

"+--+"

.

Code is from: https://discuss.leetcode.com/topic/27287/short-java-ruby?show=64350

public boolean canWin(String s) {
for (int i=-1; (i = s.indexOf("++", i+1)) >= 0; )
if (!canWin(s.substring(0, i) + "-" + s.substring(i + 2))) return true;
return false;
}