Valid Parentheses Problems

Valid Parentheses

• class Solution {
      public boolean isValid(String s) {
          if (s.length() % 2 != 0)    return false;
  
          Deque<Character> stack = new ArrayDeque<>();
  
          for (char c : s.toCharArray()) {
              switch(c) {
                  case '(', '[', '{'  -> stack.push(c);
                  case ')' -> {
                      if (stack.isEmpty() || stack.peek() != '(')
                          return false;
                      stack.pop();
                  }
                  case ']' -> {
                      if (stack.isEmpty() || stack.peek() != '[')
                          return false;
                      stack.pop();
                  }
                  case '}' -> {
                      if (stack.isEmpty() || stack.peek() != '{')
                          return false;
                      stack.pop();
                  }
              }
          }
  
          return stack.isEmpty();
      }
  }

Minimum Add to Make Parentheses Valid

• class Solution {
      public int minAddToMakeValid(String s) {
          int a = 0, b = 0;
          for (char c : s.toCharArray()) {
              if (c == '(')   a++;
              else {
                  if (a != 0) a--;
                  else        b++;
              }
          }
          return a + b;
      }
  }

Remove Outermost Parentheses

• class Solution {
      public String removeOuterParentheses(String s) {
          int count = 0, start = 0, end = 0;
          StringBuilder sb = new StringBuilder();
          char[] c = s.toCharArray();
  
          for (int i = 0; i < c.length; i++) {
              if (count == 0) {
                  if (start < end)    sb.append(s, start, end);
                  start = i + 1;
              }
              if (c[i] == '(') {
                  count++;
              }
              else {
                  count--;
                  if (count == 0)     end = i;
              }
          }
          if (start < end)    sb.append(s, start, end);
          return sb.toString();
      }
  }