Java 算法（背包，队列和栈）

Dijkstra的双栈算术表达式求值算法：

 import java.util.*;
 public class Main
 {
     public static double evaluate(String abs) {
         Stack<String> ops = new Stack<String>();
         Stack<Double> vals = new Stack<Double>();
         char arr[]=abs.toCharArray();
         for(char c:arr){
             String s=c+"";
            if(s.equals(""));
            else if(s.equals("("));
            else if (s.equals("+")) ops.push(s);
            else if (s.equals("-")) ops.push(s);
            else if (s.equals("*")) ops.push(s);
            else if (s.equals("/")) ops.push(s);
            else if (s.equals(")"))
            {
                String op=ops.pop();
                double v=vals.pop();
                if(op.equals("*")) v=vals.pop()*v;
                else if(op.equals("+")) v=vals.pop()+v;
                else if(op.equals("-")) v=vals.pop()-v;
                else if(op.equals("/")) v=vals.pop()/v;
                vals.push(v);
            }
            else vals.push(Double.parseDouble(s));
         }
         return vals.pop();
     }
     public static void main(String args[])
     {
         Scanner s=new Scanner(System.in);
         String s1=s.next();
         System.out.print(evaluate(s1));
     }
 }

下压栈（LIFO)能够动态调整数组大小的实现

 import java.util.*;
 import java.lang.*;
 public class Main<Item> implements Iterable<Item> {
     private Item[] a = (Item[])new Object[1];//栈元素
     private int N = 0;//元素数量

     public boolean isEmpty(){
         return N == 0;
     }

     public int size(){
         return N;
     }

     public void resize(int max){
         Item[] temp = (Item[])new Object[max];
         for(int i = 0;i<N;i++){
             temp[i] = a[i];
         }
         a = temp;
     }

     public void push(Item item){
         if(N == a.length) resize( 2 * a.length);
         a[N++] = item;
     }

     public Item pop(){
         Item item = a[--N];
         a[N] = null;
         if(N>0 && N == a.length/4) resize(a.length/2);
         return item;
     }

    // @Override
     public Iterator<Item> iterator() {
         return new ReverseArrayIterator();
     }

     private class ReverseArrayIterator implements Iterator<Item>{
         private int i = N;
         public boolean hasNext() {
             return i > 0;
         }

         public Item next() {
             return a[--i];
         }

        // @Override
         public void remove() {
             throw new UnsupportedOperationException();
         }

     }
     public static void main(String args[])
     {
         Main<String> s=new Main<String>();
         Scanner cin=new Scanner(System.in);
         System.out.println("Input a String end with $:");
         while(cin.hasNext()){
             String item=cin.next();
             if(item.equals("$")){
                 break;
             }
             else {
                 s.push(item);
             }
         }
         // foreach语句是while语句的一种简写方式
         System.out.println("display by for:");
         for(String str : s){
             System.out.print(str+" ");
         }
         System.out.println();
         // which
         System.out.println("display by which:");
         Iterator<String> it=s.iterator();
         while(it.hasNext()){
             String i=it.next();
             System.out.print(i+" ");
         }
     }

 }