7. Reverse Integer

Given a signed 32-bit integer x, return x with its digits reversed. If the reversed value falls outside the signed 32-bit range, return 0.

Pop the last digit with % 10 and push it onto the result with ×10 +. Before pushing, check that the result can grow by one more digit without leaving the 32-bit range.

1function reverse(x: number): number {
2  let r = 0;
3  while (x !== 0) {
4    const d = x % 10; x = Math.trunc(x / 10);
5    if (r > Math.trunc((2 ** 31 - 1) / 10) || r < Math.trunc(-(2 ** 31) / 10)) return 0;
6    r = r * 10 + d;
7  }
8  return r;
9}