1524-number-of-sub-arrays-with-odd-sum.js

/**
 * @param {number[]} arr
 * @return {number}
 */
const numOfSubarrays = function(arr) {
  const n = arr.length, mod = 1e9 + 7
  let sum = 0, res = 0, oddCnt = 0, evenCnt = 0
  
  for(let i = 0; i < n; i++) {
    const cur = arr[i]
    sum += cur
    if(sum % 2 === 1) {
      res++
      res += evenCnt
      oddCnt++
    } else {
      res += oddCnt
      evenCnt++
    }
  }
  
  return res % mod
};

// another

/**
 * @param {number[]} arr
 * @return {number}
 */
const numOfSubarrays = function(arr) {
  const n = arr.length, mod = 1e9 + 7

  let oc = 0, ec = 1
  let sum = 0
  let res = 0
  for(let i = 0; i < n; i++) {
    sum += arr[i]
    if(sum % 2 === 1) {
      res += ec
      oc++
    } else {
      res += oc
      ec++
    }
  }
  
  return res % mod
};