Coursera Algorithms week2 栈和队列 练习测验: Stack with max

题目原文：

Stack with max. Create a data structure that efficiently supports the stack operations (push and pop) and also a return-the-maximum operation. Assume the elements are reals numbers so that you can compare them.

分析：

该题目要求在实现正常stack的push和pop操作外，还需要实现返回maximum的操作，显然一个数组是不够的，需要额外的数组maximums[]来从小到大存储stack内的值，每次返回maximum就返回maximums[n-1]，可满足要求。

 import java.util.Arrays;

 /**
  * @author evasean www.cnblogs.com/evasean/
  */
 public class StackWithMax {
     public double[] items;// 为了方便展示结果，我也懒得写遍历方法，就设为public
     public double[] maximums; // 存放最大值的数组，为了方便展示结果，我也懒得写遍历方法，就设为public
     private int n;
     private int cap;

     public StackWithMax() {
         n = 0;
         cap = 2;
         items = new double[cap];
         maximums = new double[cap];
     }

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

     public void push(double item) {
         if (n == 0)
             maximums[0] = item;
         else {
             int i;
             int j;
             for (i = n - 1; i >= 0; i--) {// 这个循环用来找出item在maximums数组中应该放置的位置
                 if (item <= maximums[i])
                     continue;
                 else
                     break;
             }
             for (j = n - 1; j > i; j--) {// 将位置i以后的元素都往后挪一个位置
                 maximums[j + 1] = maximums[j];
             } // 循环结束时j指向i
             maximums[j + 1] = item;// j+1就是item应该放置的位置
         }
         items[n++] = item;
         if (n == cap)
             resize(2 * cap);
     }

     public double pop() {
         double item = items[--n];
         if (n > 0 && n == cap / 4)
             resize(cap / 2);
         return item;
     }

     public double maximum() {
         return maximums[n - 1];
     }

     private void resize(int cap) {
         double[] t1 = new double[cap];
         double[] t2 = new double[cap];
         for (int i = 0; i < n; i++) {
             t1[i] = items[i];
             t2[i] = maximums[i];
         }
         items = t1;
         maximums = t2;
         this.cap = cap;
     }

     public static void main(String[] args) {
         StackWithMax stack = new StackWithMax();
         stack.push(9);
         stack.push(8);
         stack.push(11);
         stack.push(0);
         stack.push(-9.9);
         stack.push(88);
         System.out.println(Arrays.toString(stack.items));
         System.out.println(Arrays.toString(stack.maximums));
         stack.pop();
         System.out.println(stack.maximum());
     }
 }