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- public boolean isValid(String s) {
- Stack<Character> symbols = new Stack<>();
- if (s.length() % 2 != 0) {
- return false;
- }
- for (char symb : s.toCharArray()) {
- if (symb == '(' || symb == '[' || symb == '{') {
- symbols.push(symb);
- } else {
- if (symbols.isEmpty()) return false;
- if (symb == ')' && symbols.peek() == '(') {
- symbols.pop();
- } else if (symb == ']' && symbols.peek() == '[') {
- symbols.pop();
- } else if (symb == '}' && symbols.peek() == '{') {
- symbols.pop();
- } else {
- return false;
- }
- }
- }
- return symbols.isEmpty();
- }
- public int evalRPN(String[] tokens) {
- Stack<Integer> itemSt = new Stack<>();
- int first;
- int second;
- for (String item : tokens) {
- if ("+".equals(item)) {
- itemSt.push(itemSt.pop() + itemSt.pop());
- } else if ("-".equals(item)) {
- second = itemSt.pop();
- first = itemSt.pop();
- itemSt.push(first - second);
- } else if ("*".equals(item)) {
- itemSt.push(itemSt.pop() * itemSt.pop());
- } else if ("/".equals(item)) {
- second = itemSt.pop();
- first = itemSt.pop();
- itemSt.push(first / second);
- } else {
- itemSt.push(Integer.parseInt(item));
- }
- }
- return itemSt.pop();
- }
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