20. Valid Parentheses

问题

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

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

思路

这道题很简单,就是一个经典的栈的例子——表达式中的括号符号匹配。

  • 遇见了左括号就进栈
  • 遇见了右括号就出栈
    • 如果栈为空,出错
    • 如果出栈元素不是匹配的括号,出错

这里解决出括号匹配用了一个小tick,就是利用ASCII码,匹配的括号的ascii码都不会相差太远

  • '(' ')' 相差1
  • '[' ']' '{' '}' 相差2
public class Solution {
    public boolean isValid(String s) {
        if(s.length() == 0)
            return false;
        Stack<Character> stack = new Stack<Character>(); // 创建堆栈对象 
        for(int i = 0;i < s.length(); i++)
        {
            if(s.charAt(i) == '(' || s.charAt(i) == '[' || s.charAt(i) == '{')
                stack.push(s.charAt(i));
            if(s.charAt(i) == ')' || s.charAt(i) == ']' || s.charAt(i) == '}')
            {
                if(stack.empty()) return false;
                char out = stack.pop();
                if(out - s.charAt(i) > 2)
                    return false;
            }
        }
        if(stack.empty())
            return true;
        return false;
    }
}

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