This problem is pretty straight forward.
Given an array of heights, find the two indices that could contain the most water between them.
Generally there are two ways of solving this, the brute force method, and the two pointer method.
Brute Force
This method will calculate every possible combination possible to determine the answer. This requires a nested loop resulting in a complexity of O(n^2).
Javascript
var maxArea = function(height) {
let mostWater = 0
for (let l = 0; l < height.length - 1; l++) {
for (let r = height.length - 1; r > l; r--) {
mostWater = Math.max(mostWater, (Math.min(height[l], height[r])*(r-l)))
}
}
return mostWater
}
A better way of doing this is to utilize two pointers.
Two Pointer Method
For this method we utilize two pointers that are placed on opposite ends of the array to iterate through once resulting in a time complexity of O(n). The area between the current left (l) and right (r) indexes and if the area is larger than the current max, it is set as max.
The indices are moved based on which one is smaller or if equal the left one in this situation. It is arbitrary which is checked in the if statement.
Javascript
var maxArea = function(height) {
let max = 0
let l = 0
let r = height.length - 1
while (l < r) {
if (height[l] > height[r]) {
max = Math.max(height[r] * (r-l), max)
r--
} else {
max = Math.max(height[l] * (r-l), max)
l++
}
}
return max
};
In Ruby...
def max_area(height)
max_area = 0
l_idx = 0
r_idx = height.length - 1
while (r_idx > l_idx)
if height[r_idx] >= height[l_idx]
max_area = [(height[l_idx] * (r_idx - l_idx)), max_area].max
l_idx += 1
else
max_area = [(height[r_idx] * (r_idx - l_idx)), max_area].max
r_idx -= 1
end
end
return max_area
end
For more readability you can always separate out the max_area (ruby) or max (javascript) into multiple lines.
Javascript
if (height[l] > height[r]) {
maybeMax = height[r] * (r-l)
r--
} else {
maybeMax = height[l] * (r-l)
l++
}
max = Math.max(maybeMax, max)
Ruby
if height[r_idx] >= height[l_idx]
maybe_max = height[l_idx] * (r_idx - l_idx)
l_idx += 1
else
maybe_max = height[r_idx] * (r_idx - l_idx)
r_idx -= 1
end
max_area = maybe_max if maybe_max > max_area
Decided to mix up the syntax on the last line for variety since the Ruby and JS solutions look so similar.
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