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. 

給定一個只包括 ‘(‘，’)’，’{‘，’}’，’[‘，’]’ 的字串，判斷字串是否有效。有效字串需滿足：左括號必須用相同型別的右括號閉合。左括號必須以正確的順序閉合。

public class Solution {
	public boolean isValid(String s) {
		Stack<Character> stack = new Stack<>();
		for (int i = 0; i < s.length(); i++) {
			char c = s.charAt(i);
			if (c == '(' || c == '[' || c == '{')
				stack.push(c);
			else {
				if (stack.isEmpty())
					return false;
				char topChar = stack.pop();
				if (c == ')' && topChar != '(')
					return false;
				if (c == ']' && topChar != '[')
					return false;
				if (c == '}' && topChar != '{')
					return false;
			}
		}
		return stack.isEmpty();
	}
}
 

測試程式碼

public class Test {
	public static void main(String[] args) {
		System.out.println((new Solution()).isValid("()[]{}"));
		System.out.println((new Solution()).isValid("([)]"));
	}
}

執行結果

true
false