20.valid-parentheses.java

/*
 * @lc app=leetcode id=20 lang=java
 *
 * [20] Valid Parentheses
 *
 * https://leetcode.com/problems/valid-parentheses/description/
 *
 * algorithms
 * Easy (35.92%)
 * Total Accepted:    534K
 * Total Submissions: 1.5M
 * Testcase Example:  '"()"'
 *
 * Given a string 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.
 *
 *
 * Note that an empty string is also considered valid.
 *
 * Example 1:
 *
 *
 * Input: "()"
 * Output: true
 *
 *
 * Example 2:
 *
 *
 * Input: "()[]{}"
 * Output: true
 *
 *
 * Example 3:
 *
 *
 * Input: "(]"
 * Output: false
 *
 *
 * Example 4:
 *
 *
 * Input: "([)]"
 * Output: false
 *
 *
 * Example 5:
 *
 *
 * Input: "{[]}"
 * Output: true
 *
 *
 */
class Solution {
  public boolean isValid(String s) {
    Stack<Character> stack = new Stack<>();
    for (int i = 0; i < s.length(); i++) {
      char c = s.charAt(i);
      if (c == '(') {
        stack.push(')');
      } else if (c == '{') {
        stack.push('}');
      } else if (c == '[') {
        stack.push(']');
      } else if (stack.empty() || stack.pop() != c) {
        return false;
      }
    }
    return stack.empty();
  }
}