Given two arrays A and B, return the indices at which the two arrays intersect. If the two arrays have no intersection at all, return null.

findIntersection.js

// Brute force - O(n^2)
const findIntersection = (arr1, arr2) => {
  for (let i = 0; i < arr1.length; i++) {
      for (let j = 0; j < arr2.length; j++) {
          if (arr1[i] === arr2[j]) {
              return [i, j]
          }
      }
  }
  return null
}

// Optimized solution - O(n)
const findIntersection = (arr1, arr2) => {
    let store = {}
    for (let i = 0; i < arr1.length; i++) {
        store[arr1[i]] = i
    }
    for (let j = 0; j < arr2.length; j++) {
        if (arr2[j] in store) {
           return [store[arr2[j]], j]
        }
    }
    return null
}

// Provided the arrays are sorted - O(n)
const findIntersection = (arr1, arr2) => {
    let i = 0, j = 0;
    while (i < arr1.length && j < arr2.length) {
        if (arr1[i] === arr2[j]) {
            return [i, j]
        } else if (arr1[i] < arr2[j]) {
            i++
        } else {
            j++
        }
    }
    return null
}