20.Valid Parentheses

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Question:

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:

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Input: "()"
Output: true

Example 2:

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

Example 3:

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Input: "(]"
Output: false

Example 4:

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Input: "([)]"
Output: false

Example 5:

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Input: "{[]}"
Output: true

My Answer:

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class Solution {
    public boolean isValid(String s) {
        if(s.length()==0)
            return true;
        if(s.length()==1 ||s.charAt(0)==')' || s.charAt(0)==']' ||s.charAt(0)=='}')
            return false;       
        Stack<Character> stack = new Stack<Character>(); 
        for(int i=0;i<s.length();i++){
            char c = s.charAt(i);
            if(c =='(' || c =='[' || c =='{'){
                stack.push(c);
            }
            else if(c == ')' && !stack.empty()){
                if(stack.pop() != '(')
                    return false;                            
            }
            else if(c == ']' && !stack.empty()){
                if(stack.pop() != '[')
                    return false;                
            }
            else if(c == '}' && !stack.empty()){                
                if(stack.pop() != '{')
                    return false; 
            }
            else 
                return false;
        }
        return stack.empty();
    }
}

Running time: 8ms

Better Answer 1:

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public static boolean isValid(String s) {
      if (s.isEmpty()) return true;
      Stack<Character> st = new Stack<>();
      for (char c : s.toCharArray()) {
          if (st.isEmpty()) st.push(c);
          else {
              char last = st.peek();
              if (c == '(' || c == '{' || c == '[') st.push(c);
              else if (c == ')' && last == '(' || c == '}' && last == '{'
                      || c == ']' && last == '[') st.pop();
              else return false;
          }
      }
      return st.isEmpty();
  }

Better Answer 2:

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public boolean isValid(String s) {
	Stack<Character> stack = new Stack<Character>();
	for (char c : s.toCharArray()) {
		if (c == '(')
			stack.push(')');
		else if (c == '{')
			stack.push('}');
		else if (c == '[')
			stack.push(']');
		else if (stack.isEmpty() || stack.pop() != c)
			return false;
	}
	return stack.isEmpty();
}