20.Valid-Parentheses

20. Valid Parentheses

题目地址

https://leetcode.com/problems/valid-parentheses/

题目描述

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

An input string is valid if:

Open brackets must be closed by the same type of brackets.
Open brackets must be closed in the correct order.
Note that an empty string is also considered valid.

代码

Approach 1: Stacks

Complexity analysis:

• Time complexity: O(n)

• Space complexity: O(n)

public class Solution {
    public boolean isValid(String s) {
    HashMap<Character, Character> mappings = new HashMap<Character, Character>();
    mappings.put(')', '(');
    mappings.put('}', '{');
    mappings.put(']', '[');

    Stack<Character> stack = new Stack<Character>();

    for (int i = 0; i < s.length(); i++) {
      char c = s.charAt(i);

      if (mappings.containsKey(c)) {
        char topElement = stack.empty() ? '#' : stack.pop();
        if (topElement != mappings.get(c)) return false;
       } else {
          stack.push(c);
      }
    }

    return stack.isEmpty();
  }
}