leetcode刷題Valid Parentheses

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

Input: "()[]{}"
Output: true

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

 

分析題目可以發現,這裏面的數據都是有一定的對稱性,有一種各自匹配消除的感覺,這裏就想到了棧, 由於{},[]和()的ASSCII碼的差值還不一樣,前面兩個是2,後面一個是1,這個要注意一下.

這道題目的主要目的是檢測測試者對棧的數據結構的熟悉程度,這道題目用棧做很方便.直接上代碼

if(s.empty())
               return true;
           int num = s.size();
           int mid = int(num / 2);
           if(num%2 !=0)
               return false;
           stack <int> all;
           for(int i=0;i<num;i++)
           {

                   if(!all.empty()&&((all.top()-s[i] == -1)||(all.top()-s[i]==-2)))
                   {
                       
                          all.pop();
                   }
                   else
                   {
                       all.push(s[i]);
                   }

           }

           if(all.empty())
              return true;
           else
               return false;
    }

note:  char可以強制轉換成int型

note:在運用A&& B判斷時,只要A不滿足條件,條件B是不執行的,所以不用擔心棧的pop會越界.這個小知識點很有用