Container With Most Water

Leetcode problem: https://leetcode.com/problems/container-with-most-water/

Problem Statement

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.

Example 1:

Input: height = [1,8,6,2,5,4,8,3,7]
Output: 49
Explanation: The above vertical lines are represented by array [1,8,6,2,5,4,8,3,7]. In this case, the max area of water (blue section) the container can contain is 49.

Example 2:

Input: height = [1,1]
Output: 1

Constraints:

• n == height.length

• 2 <= n <= 105

• 0 <= height[i] <= 104

Note

The area is bound by the smaller height of the two lines. (I hope it's logical to you 😌)

Discussion

Concepts

• Two pointers - Notice that there's a left and a right in this problem? Yupp, usually when there's a left and a right, two pointers can be used. Two pointers is a general algorithm that helps to solve the problem by keeping track and moving a left & a right pointers repetitively.

• Greedy Algorithm - This involves how the two pointers are moving. Basically, we will always choose the taller line. Greedy here refers to taking the current tallest (taller of the two) and assuming it's the best now, instead of searching for the tallest of all. Details will be discussed in the idea section.

Brute Force Attempt

I'll skip the codes, but it'll be almost identical to the brute force from the previous problem.

The brute force here will involve computing the area of every pair of lines, and getting the maximum of them.

The complexity of brute force will be \(O(n^2)\) from the iteration of every pair (due to nested loop) as area calculation is only \(O(1)\).

Optimal Approach

Idea

The optimal approach (greedy algorithm) involves moving the two pointers from outside in.

We will start our two pointers from leftmost and rightmost.

Why? Well, as we know, area = width * height. Thus, the height and width are what we are trying to maximize here. So I start from the maximum width and try to get the best height along the way.

While moving, we will compute the area and compare for the maximum in every iteration.

def calculateArea(heights, left, right):
    return min(heights[left], heights[right]) * (right - left)

Next question is, how do we decide whether we move leftPointer or rightPointer?

As I said, we try to maximize the width and height. Now that we're at maximum width, we will search for the best height. How? We move the pointer with lower height.

That's it! We will repeat this and we will get the greatest height in the end.

Discussion

Before we begin, here's some notation

bestLeft, bestRight = the lines where the maximal area, maximalArea lies in

Just an additional discussion section in case you have some questions that I had:

1. Why don't we go inside out but outside in?

   Well, there's only one problem with this, where do we start?
   To get the maximum area, we must ensure that our leftPointer will eventually reach bestLeft and the same for our bestRight.

   Say we decided to start from center

   

   By expanding our leftPointer left, it'll never reach bestLeft, hence failing the algorithm.

2. Why does this work?

   Let's ask it the other way around, why does this not work? (proof by contradiction)

   Using our greedy algorithm, there's one thing for sure, either our leftPointer will eventually touch bestLeft or rightPointer will eventually touch bestRight.

   

   Notice that leftPointer always moves width - steps by rightPointer before they meet up.

   So let's say our rightPointer is already at bestRight, while our leftPointer has not touched bestLeft yet.

   

   So in what scenario our leftPointer doesn't touch bestLeft? When rightPointer reaches bestLeft, that is, we need to move our rightPointer from bestRight.

   And when are we moving our rightPointer? When leftPointer > rightPointer, or leftPointer > bestRight.

   

   However, if that is the case, there are contradictions here:

   1. if bestRight <= bestLeft,

      $Area = bestRight * width$ (since bestRight is the shorter of two)

      

      and since leftPointer is on the left of bestLeft, the width here is larger. This means that the area here is larger than maximalArea.

   2. if bestRight > bestLeft,

      $Area = bestLeft * width$

      

      and since leftPointer > bestRight > bestLeft, doesn't this mean that the area from leftPointer and bestRight is bigger than maximalArea?

   Thus, the contradiction shows that the scenario will not happen, i.e., leftPointer will always reach leftBest and rightPointer will always reach rightBest.

   Reference from leetcode
   Of course, there are many more amazing explanations out there, do check them out if you're not convinced by this! (at least I'm convinced by this)

Codes

def calculateArea(heights, left, right):
    return min(heights[left], heights[right]) * (right - left)

class Solution:
    def maxArea(self, height: List[int]) -> int:
        curMax = 0
        left = 0
        right = len(height) - 1
        while left < right:
            curMax = max(curMax, calculateArea(height, left, right))
            ## It's the same using < or <=
            if height[left] <= height[right]:
                left += 1
            else:
                right -= 1
                
        return curMax

Notice I mentioned that it doesn't matter using < or <=, this is because if leftPointer == rightPointer,

Regardless of which pointer you move, the resulting area will be smaller. (width lower, but same height)

Analysis

Time Complexity

As we will only go through the entire array once (from moving leftPointer and rightPointer) and calculating the area once for each position, the time complexity will be O(n)

As mentioned previously, calculateArea is O(1)

Space Complexity

O(1) space, as space is only allocated to pointers.

Last Words

While this problem is not too difficult in terms of algorithm, I find the proof by contradiction to be rather interesting and widened my horizon! 🤩