Problem Description:

Given a string s 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.

Link to the problem

Solution:

class Solution {
    public boolean isValid(String s) {
        String input=s;
        boolean flag=true;
        Stack<Character> stack = new Stack<Character>();

        mainLoop:
        for(int i=0; i<input.length(); i++) {
            char ch = input.charAt(i);
            switch(ch) {
                case '(':
                case '{':
                case '[':
                    stack.push(ch);
                    break;
                case ')':
                    if(!stack.empty()){
                        Character top = (Character)stack.peek();
                        if(top.compareTo('(') == 0) {
                            stack.pop();
                        } else {
                            flag=false;
                            break mainLoop;
                        }
                    } else {
                        flag=false;
                        break mainLoop;
                    }
                    break;
                case '}':
                    if(!stack.empty()){
                        Character top = (Character)stack.peek();
                        if(top.compareTo('{') == 0) {
                            stack.pop();
                        } else {
                            flag=false;
                            break mainLoop;
                        }
                    } else {
                        flag=false;
                        break mainLoop;
                    }
                    break;
                case ']':
                    if(!stack.empty()){
                        Character top = (Character)stack.peek();
                        if(top.compareTo('[') == 0) {
                            stack.pop();
                        } else {
                            flag=false;
                            break mainLoop;
                        }
                    } else {
                        flag=false;
                        break mainLoop;
                    }
                    break;

            }
        }
        if (!stack.empty()){
            flag=false;
        }    
        return flag;
    }
}