Index Difference of Tuples in JavaScript

Problem

We are required to write a JavaScript function that takes in an array of integers, arr, as the first and the only argument.

Suppose two indices, i and j in the array which satisfy the following conditions −

• i < j, and

• arr[i] <= arr[j]

Out of all such index tuples (i, j), our function should return the difference j - i, where it is the maximum.

For example, if the input to the function is −

const arr = [6, 0, 8, 2, 1, 5];

Then the output should be −

const output = 4;

Output Explanation

The maximum difference is achieved at (i, j) = (1, 5): arr[1] = 0 and arr[5] = 5.

Example

The code for this will be −

 Live Demo

const arr = [6, 0, 8, 2, 1, 5];
const maximumDifference = (arr = []) => {
   let max = 0
   const stack = [0]
   for (let i = 1; i < arr.length; i++) {
      if (arr[i] < arr[stack[stack.length - 1]]) {
         stack.push(i)
      }
   }
   for (let i = arr.length - 1; i >= 0; i--) {
      while (arr[i] >= arr[stack[stack.length - 1]]) {
         max = Math.max(max, i - stack.pop())
      }
   }
   return max;
};
console.log(maximumDifference(arr));

Output

And the output in the console will be −

4