Teaching Kids Programming: Videos on Data Structures and Algorithms
You are given an integer array height of length n. There are n vertical lines drawn such that the two endpoints of the ith line are (i, 0) and (i, height[i]).
Find two lines that together with the x-axis form a container, such that the container contains the most water.
Return the maximum amount of water a container can store.
Notice that you may not slant the container.
Bruteforce Algorithm to Find the Two Vertical Bars that Holds Most Water
Given there are N bars, we want to find the optimal two – a hint to use bruteforce algorithms. There are ways which indicates time complexity.
The volume of water is determined by the shortest bar and we can easily compute the area of a rectangle.
1 2 3 4 5 6 7 8 | class Solution: def maxArea(self, height: List[int]) -> int: n = len(height) ans = 0 for L in range(n): for R in range(L, n): ans = max(ans, min(height[L], height[R]) * (R - L)) return ans |
class Solution: def maxArea(self, height: List[int]) -> int: n = len(height) ans = 0 for L in range(n): for R in range(L, n): ans = max(ans, min(height[L], height[R]) * (R - L)) return ans
Two Pointer Algorithm to Find the Two Vertical Bars that Holds Most Water
Two Pointer can usually be used to improve O(N^2) to O(N). We initialize the two pointers left and right – which is located at the left-most and right-most bars. We then update the max volume and move the shorter bar towards the other (Greedy Strategy).
1 2 3 4 5 6 7 8 9 10 11 12 13 | class Solution: def maxArea(self, height: List[int]) -> int: n = len(height) ans = 0 L = 0 R = n - 1 while L < R: ans = max(ans, min(height[L], height[R]) * (R - L)) if height[L] <= height[R]: L += 1 else: R -= 1 return ans |
class Solution: def maxArea(self, height: List[int]) -> int: n = len(height) ans = 0 L = 0 R = n - 1 while L < R: ans = max(ans, min(height[L], height[R]) * (R - L)) if height[L] <= height[R]: L += 1 else: R -= 1 return ans
Proof of Two Pointer Algorithms
Current volume is where t is the distance between L and R. If we move the longer bar which has a new height R1, and the distance t1, we get:
.
We know and
Therefore, we don’t get a larger volume by moving the longer bar towards the shorter bar – then we don’t need to bruteforce those situations. Instead, we can skip and move the shorter bar.
The two pointer has O(N) time complexity.
–EOF (The Ultimate Computing & Technology Blog) —
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