1003. Check If Word Is Valid After Substitutions.java

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