code: function findLastCommonIndex(xs, ys) {
      for (let i = Math.min(xs.length, ys.length) - 1; i >= 0; --i) {
          if (xs[i] === ys[i]) {
              return i
          }
      }
      return -1
  }
docs: Get the index of the last element which is same between the given 2 elements.
  
  @param {string[]} xs - An array to compare.
  @param {string[]} ys - Another array to compare.
  @returns {number} The index of the last element is same.
  @private