SOURCE

function  get(m,n,k) {
  // 求边界值【无法跳出的最大边界值】，在提示前提k<=20的算式
  let max = k<8?k:(k - 8 < 10 ? ((k - 8) * 10 + 8) : ((k - 9 - 8) * 100 + 98))
  // console.log(max)
  let rsnum = 0
  for (let i = 0; i <= Math.min(m-1,max); i++) {
    // x轴求和
    let xsum = 0
    i.toString().split('').forEach(num => {
      xsum += parseInt(num)
    });
    for (let j = 0; j <= Math.min(n-1, max-i);j++){
      // y轴求和
      let ysum = 0
      j.toString().split('').forEach(num => {
        ysum += parseInt(num)
      });
      xsum + ysum <= k && rsnum++
        // console.log(`x:${i},y:${j},sum:${xsum + ysum}`)
    }
  }
  console.log(rsnum)
  return rsnum
}

get(38,15,9)
get(100,100,9)