specialParis.js

// Given an array of integers arr, a pair (n,m) is called “special” if arr[n] == arr[m], and n < m.
// Return the number of special pairs.
// Hint: Nested for loops can work for this one, but a hashmap solution will have a better runtime!

// $ specialPairs([1,2,3,1,1,3])
// $ 4 // (0,3), (0,4), (3,4), (2,5)
function specialPairs(arr) {
  // let cache = {};
  let count = 0;

  for (let i = 0; i < arr.length; i++) {
    for (let j = i + i; j < arr.length; j++) {
      if (arr[i] === arr[j]) {
        count += 1;
      }
    }
  }

  return count;
}

function specialPairsReduce(arr) {
  let cache = arr.reduce((prev, curr, idx) => {
    prev[curr] = Array.isArray(prev[curr]) ? prev[curr].concat([idx]) : [idx];
    return prev;
  }, {});

  let count = 0;
  Object.keys(cache).forEach((el) => {
    if (cache[el].length > 1) {
      if (cache[el].length % 2 === 0) {
        count += cache[el].length;
      } else {
        count += cache[el].length - 1;
      }
    }
  });

  return count;
}

console.log(specialPairsReduce([1, 2, 3, 1, 1, 3, 1]));