Leet Code 20 Valid Parentheses

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.

【算法思路】

用栈。

public boolean isValid(String s) {int length = s.length();if(length % 2 == 1){return false;}HashMap<Character, Character> map = new HashMap<Character, Character>();map.put('(', ')');map.put('{', '}');map.put('[', ']');Stack<Character> stack = new Stack<Character>();if(map.containsKey(s.charAt(0)))    stack.push(s.charAt(0));    else        return false;for(int i = 1; i < length; i ++){char ch = s.charAt(i);if(!stack.isEmpty() && (map.get(stack.peek()) == ch)){stack.pop();}else if(map.containsKey(ch))stack.push(ch);else return false;if(!stack.isEmpty() && (stack.size() > length - i))return false;}return stack.isEmpty();}