public int water(int[] height) { int n = height.length, sum = 0; int[] right = new int[n]; int rightMax = height[n - 1]; for (int i = n - 1; i >= 0; i--) { right[i] = rightMax; rightMax = Math.max(rightMax, height[i]); } int leftMax = height[0]; for (int i = 0; i < n; i++) { int min = Math.min(leftMax, right[i]); sum += min > height[i] ? min - height[i] : 0; leftMax = Math.max(leftMax, height[i]); } return sum; }