直方图最大面积

Q:给定一个直方图，这里盗一下图，http://blog.csdn.net/nisxiya/article/details/46562951，
例如

20150619153844882.jpg

的最大面积为10

烦烦烦.jpg

A：关键在于单调栈的运用，单调栈可以找到每个元素左右两边离他最近的比他小（或者大）的数。

import java.util.*;
import java.io.*;
public class 最大直方图面积 {

    public static void main(String[] args) throws Exception{
        Scanner scanner=new Scanner(System.in);
        int length = scanner.nextInt();
        int[] arr = new int[length];
        for (int i = 0; i < length; i++) {
            arr[i] = scanner.nextInt();
        }
        long maxArea = getMaxArea(arr);
        System.out.println(maxArea);
    }
    public static long getMaxArea(int[] arr) {
        long max = 0;
        int[] temp = new int[arr.length];
        Stack stack = new Stack<>();
        int preIndex;
        for (int i = 0; i < arr.length; i++) {
            while (!stack.isEmpty() && (arr[i] < arr[stack.peek()])) {
                preIndex = stack.pop();
                if(stack.isEmpty()){
                    temp[preIndex] = i -preIndex;
                }else{
                    temp[preIndex] = i - stack.peek()-1;
                }
            }
            stack.push(i);
        }
        int end=arr.length-1;
        while(!stack.isEmpty()){
            preIndex = stack.pop();
            if(stack.isEmpty()){
                temp[preIndex] = 1;
            }else{
                temp[preIndex] = end - stack.peek();
            }
        }
        for (int i = 0; i < arr.length; i++) {
            max = Math.max(max, arr[i] * temp[i]);
        }
        return max;
    }
}