算法Sedgewick第四版-第1章基础-010一检查括号是否成对出现

 /******************************************************************************
  *  Compilation:  javac Parentheses.java
  *  Execution:    java Parentheses
  *  Dependencies: In.java Stack.java
  *
  *  Reads in a text file and checks to see if the parentheses are balanced.
  *
  *  %  java Parentheses
  *  [()]{}{[()()]()}
  *  true
  *
  *  % java Parentheses
  *  [(])
  *  false
  *
  ******************************************************************************/

 public class Parentheses {
     private static final char LEFT_PAREN     = '(';
     private static final char RIGHT_PAREN    = ')';
     private static final char LEFT_BRACE     = '{';
     private static final char RIGHT_BRACE    = '}';
     private static final char LEFT_BRACKET   = '[';
     private static final char RIGHT_BRACKET  = ']';

     public static boolean isBalanced(String s) {
         Stack<Character> stack = new Stack<Character>();
         for (int i = 0; i < s.length(); i++) {
             if (s.charAt(i) == LEFT_PAREN)   stack.push(LEFT_PAREN);
             if (s.charAt(i) == LEFT_BRACE)   stack.push(LEFT_BRACE);
             if (s.charAt(i) == LEFT_BRACKET) stack.push(LEFT_BRACKET);

             if (s.charAt(i) == RIGHT_PAREN) {
                 if (stack.isEmpty())           return false;
                 if (stack.pop() != LEFT_PAREN) return false;
             }

             else if (s.charAt(i) == RIGHT_BRACE) {
                 if (stack.isEmpty())           return false;
                 if (stack.pop() != LEFT_BRACE) return false;
             }

             else if (s.charAt(i) == RIGHT_BRACKET) {
                 if (stack.isEmpty())             return false;
                 if (stack.pop() != LEFT_BRACKET) return false;
             }
         }
         return stack.isEmpty();
     }

     public static void main(String[] args) {
         In in = new In();
         String s = in.readAll().trim();
         StdOut.println(isBalanced(s));
     }
 }

 /*************************************************************************
  *
  * % java Ex_1_3_04
  * [()]{}{[()()]()}
  * true
  *
  * % java Ex_1_3_04
  * [(])
  * false
  *
  *************************************************************************/

 public class Ex_1_3_04
 {
     public static boolean isBalanced(String s)
     {
         Stack<Character> open = new Stack<Character>();
         int n = s.length();

         for (int i = 0; i < n; i++)
         {
             char c = s.charAt(i);

             if (c == '(' || c == '[' || c == '{')
                 open.push(c);
             else if ((c == ')' && (open.isEmpty() || open.pop() != '(')) ||
                      (c == ']' && (open.isEmpty() || open.pop() != '[')) ||
                      (c == '}' && (open.isEmpty() || open.pop() != '{')))
                 return false;
         }

         return open.isEmpty();
     }

     public static void main(String[] args)
     {
         String s = StdIn.readAll().trim();

         StdOut.println(isBalanced(s));
     }
 }