【leetcode】1047. Remove All Adjacent Duplicates In String

题目如下：

Given a string S of lowercase letters, a duplicate removal consists of choosing two adjacent and equal letters, and removing them.

We repeatedly make duplicate removals on S until we no longer can.

Return the final string after all such duplicate removals have been made.  It is guaranteed the answer is unique.

Example 1:

Input: "abbaca"
Output: "ca"
Explanation: 
For example, in "abbaca" we could remove "bb" since the letters are adjacent and equal, and this is the only possible move.  The result of this move is that the string is "aaca", of which only "aa" is possible, so the final string is "ca".

Note:

1. 1 <= S.length <= 20000
2. S consists only of English lowercase letters.

解题思路：注意两个相同的元素相邻就能消掉，三个相同的元素相邻还会剩下一个。解法也很简单，遍历Input，如果当前字符和前一个字符一样，则两者都消除。

代码如下：

class Solution(object):
    def removeDuplicates(self, S):
        """
        :type S: str
        :rtype: str
        """
        stack = []
        for i in S:
            if len(stack) == 0 or i != stack[-1]:
                stack.append(i)
            else:
                del stack[-1]
        return ''.join(stack)