表达式求值--栈

　　用栈的方式，将中缀转为后缀，再进行求值。另外附加括号匹配的方法

  1 public class PostfixExpression {
  2     
  3     public static int lastvalue(String str){    //后缀表达式求值,传入空格分割的字符串
  4         Stack<Integer> stack = new Stack<Integer>();
  5         String[] data = str.split(" ");
  6         
  7         for(int i = 0;i < data.length;i++){
  8             if(data[i].charAt(0) >= '0' && data[i].charAt(0) <= '9'){
  9                 //将data[i]转换为int数据
 10                 int m =  Integer.parseInt(data[i]);
 11                 stack.push(m);
 12             }else{
 13                 int a = stack.pop();
 14                 int b = stack.pop();
 15                 //构造b ? a计算结果,，将结果入栈
 16                 switch(data[i].charAt(0)){
 17                     case '+':stack.push(b+a);break;
 18                     case '-':stack.push(b-a);break;
 19                     case '*':stack.push(b*a);break;
 20                     case '/':stack.push(b/a);break;
 21                     case '%':stack.push(b%a);break;
 22                 }
 23             }
 24 //            System.out.println("---list:"+stack);
 25         }
 26         return stack.peek();
 27     }
 28     public static String turnvalue(String s){    //中缀变换后缀
 29         Stack<String> stack = new Stack<String>();
 30         StringBuffer sb = new StringBuffer();
 31         int i = 0;
 32         while(i < s.length()){
 33             char c = s.charAt(i);
 34             switch(c){
 35                 case '+':case '-':
 36                     while(!stack.isEmpty() && !stack.peek().equals("("))
 37                         sb.append(stack.pop()+" ");
 38                     stack.push(c + "");i++;
 39                     break;
 40                 case '*':case '/':
 41                     while(!stack.isEmpty() && (stack.peek().equals("*") || stack.peek().equals("/")))
 42                         sb.append(stack.pop()+" ");
 43                     stack.push(c + "");i++;
 44                     break;
 45                 case '(':
 46                     stack.push(c + "");i++;
 47                     break;
 48                 case ')':
 49                     while(stack.peek() != null && !stack.peek().equals("("))
 50                         sb.append(stack.pop() + " ");
 51                     stack.pop();
 52                     i++;break;
 53                 default:
 54                     if(c == ';'){
 55                         i++;
 56                         break;
 57                     }
 58                     while(i < s.length() && c >='0' && c <= '9'){
 59                         sb.append(c);i++;
 60                         if(i < s.length())
 61                             c = s.charAt(i);
 62                     }
 63                     sb.append(" ");
 64             }
 65         }
 66         while(!stack.isEmpty())
 67             sb.append(stack.pop()+" ");
 68 //        System.out.println(sb);
 69         
 70         return sb.toString();
 71     }
 72     public static String match(String str) {    //计算括号匹配
 73         int x = 0; // 花括号的个数
 74         int y = 0; // 方括号的个数
 75         int z = 0; // 弧括号的个数
 76         int k = 0; // 尖括号的个数
 77         boolean isMatch = true; // 是否匹配
 78         Stack<Character> stack = new Stack<Character>();
 79 
 80         if( str.charAt(0) == ')')
 81             isMatch = false;
 82         for (int i = 0; i < str.length(); i++) {
 83             char ch = str.charAt(i);
 84             stack.push(ch);
 85             if (ch == '}') {
 86                 x++;
 87                 while(isMatch){
 88                     if (stack.isEmpty()){
 89                         isMatch = false;
 90                         break;
 91                     }
 92                     if(stack.pop() == '{')
 93                         break;
 94                 }
 95             }
 96             if (ch == ']') {
 97                 y++;
 98                 while(isMatch){
 99                     if (stack.isEmpty()){
100                         isMatch = false;
101                         break;
102                     }
103                     if(stack.pop() == '[')
104                         break;
105                 }
106             }
107             if (ch == ')') {
108                 z++;
109                 while(isMatch){
110                     if (stack.isEmpty()){
111                         isMatch = false;
112                         break;
113                     }
114                     if(stack.pop() == '(')
115                         break;
116                 }
117             }
118             if (ch == '>') {
119                 k++;
120                 while(isMatch){
121                     if (stack.isEmpty()){
122                         isMatch = false;
123                         break;
124                     }
125                     if(stack.pop() == '<')
126                         break;
127                 }
128             }
129         }
130 //        System.out.println("list:"+stack);
131         if(!stack.isEmpty())
132             isMatch = false;
133 
134         StringBuffer sb = new StringBuffer();
135         if (isMatch)
136             sb.append("配对。共有"+ x +"对{}、"+ y +"对[]、"+ z +"对()、"+ k +"对<>。");
137         else
138             sb.append("不配对。");
139 
140         return sb.toString();
141     }
142 }