678-Valid-Parenthesis-String.java

class Solution {

    public boolean checkValidString(String s) {
        int count = 0;
        Stack<Integer> op = new Stack<>();
        Stack<Integer> st = new Stack<>();

        for (int i = 0; i < s.length(); ++i) {
            if (s.charAt(i) == '(') op.push(i); // index of opening
            else if (s.charAt(i) == ')') {
                if (op.size() > 0) op.pop(); // if we have brackets
                else if (st.size() > 0) st.pop(); // if not brackets do we have stars
                else return false; // a closing bracket without opening and star
            } else st.push(i); // index of star
        }
        // if we left with some opening bracket that over stars con cover up
        while (op.size() > 0 && st.size() > 0) {
            if (op.peek() > st.peek()) return false;
            op.pop();
            st.pop();
        }

        return op.size() == 0;
    }
}

// Time complexity: O(n)
// Space complexity: O(1)
class Solution2 {

    public boolean checkValidString(String s) {
        int low = 0, high = 0;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(') {
                low++;
                high++;
            } else if (s.charAt(i) == ')') {
                if (low > 0) {
                    low--;
                }
                high--;
            } else {
                if (low > 0) {
                    low--;
                }
                high++;
            }
            if (high < 0) {
                return false;
            }
        }
        return low == 0;
    }
}