接雨水 DP 双指针

力扣 接雨水

public class 接雨水
{
	public static int trap(int[] height)
	{
		int res = 0;
		int len = height.length;
		int[] maxLeft = new int[len];//存 i 左边最高的高度
		int[] maxRight = new int[len];//存 i 右边最高的高度
		maxLeft[0] = 0;
		maxRight[len - 1] = 0;
//		DP
		for (int i = 1; i < len; i++)
			maxLeft[i] = Math.max(height[i - 1], maxLeft[i - 1]);
		for (int i = len - 2; i > 0; i--)
			maxRight[i] = Math.max(height[i + 1], maxRight[i + 1]);
		
		for (int i = 1; i < len - 1; i++)// 注意：第一块和最后一块无需处理，因为不会积水
		{
			int min = Math.min(maxLeft[i], maxRight[i]);
			if (min > height[i])
				res += min - height[i];
		}
		return res;
	}

	public static void main(String[] args)
	{
		int[] a = { 4, 2, 0, 3, 2, 5 };
		int res = trap(a);
		System.err.print(res);
	}
}