区间最大值

题目：给定一个数组  求出截取出来的和和当前最小值的乘积结果的最大值

 

直接上答案：

/**
 * 当前最小值 设每个值为当前最小值
 * 前缀求和数组
 * 通过栈的大小比较进出，可以确认到当前最小值的最远下标，也就是最大求和
 * 升级版：顺序可以打乱
 * @param args
 */
public static void main(String[] args) {
    int[] number = new int[]{6,5,2,18,19};
    Stack<Integer> stack = new Stack<>();
    System.out.println(getMax(number));
}

public static int getMax(int[] numbers) {
    if(numbers==null||numbers.length==0){
        return 0;
    }
    Stack<Integer> stack = new Stack<>();
    int[] sum = new int[numbers.length+1];
    int max = 0;
    for (int i = 1; i <= numbers.length; i++) {
        sum[i] = sum[i - 1] + numbers[i - 1];
    }
    for(int i = 0;i< numbers.length;i++){
        while (!stack.isEmpty() && numbers[i] < numbers[stack.peek()]){
            int index = stack.pop();
            int left = i;
            int right = i;
            if(stack.isEmpty()){
                left = 0;
            }else {
                left = index;
            }
            max = Math.max(max,numbers[index]*(sum[right]-sum[left]));
        }
        stack.push(i);
    }

    while (!stack.isEmpty()){
        int index = stack.pop();
        int left = numbers.length;
        int right = numbers.length;
        if(stack.isEmpty()){
            left = 0;
        }else {
            left =index;
        }
        max = Math.max(max,numbers[index]*(sum[right]-sum[left]));
    }

    return max;
}