接雨水-备忘录算法

算法参考：https://mp.weixin.qq.com/s?__biz=MzAxODQxMDM0Mw==&mid=2247484482&idx=1&sn=9503dae2ec50bc8aa2ba96af11ea3311&source=41#wechat_redirect

备忘录算法使用Map存储已查询过的结果，用以减少循环次数。

    public static int w() {
        int sumWater = 0;

        int[] a = {0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1};
        //左侧最大值
        int[] leftMaxArray = new int[a.length];
        //右侧最大值
        int[] rightMaxArray = new int[a.length];

        leftMaxArray[0] = a[0];
        rightMaxArray[rightMaxArray.length - 1] = 0;

        //获得左侧最大值
        for (int j = 1; j < a.length; j++) {
            leftMaxArray[j] = Math.max(leftMaxArray[j - 1], a[j-1]);
        }

        //获取右侧最大值
        for (int j = rightMaxArray.length - 2; j >= 1; j--) {
            rightMaxArray[j] = Math.max(rightMaxArray[j + 1], a[j+1]);
        }

        for (int i = 0; i < a.length - 1; i++) {
            //同时满足左侧最大值大于当前值，并且右侧最大值大于当前值，才可以接雨水
            if (leftMaxArray[i] > a[i] && rightMaxArray[i] > a[i]) {
                int temp = Math.min(leftMaxArray[i], rightMaxArray[i]) - a[i];
                sumWater += temp;
                System.out.println("i=" + i + ";值=" + a[i] + ";highLeftMax=" + leftMaxArray[i] + ";highRightMax=" + rightMaxArray[i] + ";sumWater=" + temp);
            }
        }
        System.out.println(sumWater);
        return sumWater;
    }