MEDIUM
DATA STRUCTURES AND ALGORITHMS

How to solve the Container With Most Water Problem

GRAPH THEORYARRAYSSEARCHSORTING
Written By
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Tom Wagner
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Carlos Wang

Introduction Container With Most Water

The Container with Most Water problem involves finding the largest possible area that can be formed by two vertical lines on a graph, bounded by the height of the shorter of the two lines. This problem can be solved using a two-pointer approach, which involves traversing the array from both sides and keeping track of the maximum area found so far.

Problem Container With Most Water

Given n non-negative integers a0, a1, a2, ..., a[n-1], where each represents a point at coordinate (i, a[i]), n vertical lines are drawn such that the two endpoints of the line i is at (i, 0) and (i, ai). Find two lines that, together with the x-axis, form a container that can hold the most amount of water possible.

Example Inputs and Outputs

Example

  • Input: heights = [3, 9, 4, 8, 2, 6, 1]
  • Output: 24

Constraints

  • number of integers n: [2, 10,000]
  • each integer a[i]: [0, 1,000]

Solution Container With Most Water

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Container With the Most Water Approaches

Approach 1: Brute Force

In the context of this problem, the size of the 2D container is determined by multiplying its width by its height. The brute force approach is to calculate all possible containers to find the largest one.

To accomplish this, we can iterate over the heights, forming a container with every other height to its right. Each container's size will be the lower of the two heights multiplied by the distance between the heights.

We can track the max container size as we go and return the max container size at the end of our iteration.'

Container With the Most Water Python Solution - Brute Force
def max_water(heights: list[int]) -> int:
    ln = len(heights)
    max_area = 0
    for left_index in range(ln):
        left_height = heights[left_index]
        for right_index in range(left_index + 1, ln):
            right_height = heights[right_index]
            width = right_index - left_index
            height = min(left_height, right_height)
            area = width * height
            max_area = max(area, max_area)
    return max_area
Time / Space Complexity
  • Time complexity: O(n²)
  • Space complexity: O(1)

The nested loops produce O(n²) time complexity.

Approach 2 (Optimal): Two Pointers

Although the brute force approach does not repeat any calculations, it ignores useful information that can help eliminate unnecessary calculations. For example, if a container has sides a[i] and a[j] such that a[i] < a[j], all containers with sides a[i] to a[i+1], ... a[j-1] will have a maximum height of a[i] with a smaller width than j - i, producing less area than the original container. Thus, these possibilities do not need to be considered when we're looking for maximum area.

Instead of trying every combination, we start the 2 pointers at opposite ends (indexes 0 and n-1) to represent the sides of the container. After computing the area using the lower height and distance between the pointers, the options are to increment the left pointer or decrement the right pointer.

Because we want to maximize the water contained, move the pointer with the lower height toward the other pointer. If the heights are equal, we can update either pointer because any potential increase in the next height is limited by one of the equal existing heights. The process is repeated until the pointers meet.

Container With the Most Water Python Solution - Two Pointers

_height = heights[right_index]
        min_height = min(left_height, right_height)
        area = width * min_height
        max_area = max(area, max_area)
        if left_height <= right_height:
            left_index += 1
        else:
            right_index -= 1
    return max_area

Time / Space Complexity

  • Time complexity: O(n)
  • Space complexity: O(1)

Because the left or right pointer is moved toward the other in each iteration until they meet, the list of integers is traversed once.

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