四元组之和--454

// 454: 4sum-II; 求个数
function fourSum2(numsA, numsB, numsC, numsD) {
  let map = {}
  for (let i = 0, len = numsC.length; i < len; i++) {
    let item = numsC[i]
    for (let j = 0, len = numsD.length; j < len; j++) {
      let c = list[j]
      if (typeof map[item + c] === 'undefined') {
        map[item + c] = 0
      } else {
        map[item + c]++
      }
    }
  }
  let res = 0
  for (let i = 0, len = numsA.length; i < len; i++) {
    for (let j = 0, len = numsB.length; j < len; j++) {
      let m = map[0 - numsA[i] - numsB[j]]
      if (typeof m !== 'undefined') {
        res += m
      }
    }
  }
  return res
}