why long, not int otherwise limit time exceed

Leetcode problem: valid perfect square

My answer is:

public class Solution {
    public boolean isPerfectSquare(int num) {
        int l = 0;
        int r = (num>>1)+1;
        while(l<=r) {
            int mid = (l+r)>>1;
            long temp = mid*mid;// why long
            if( temp == num) return true;
            else if( temp< num) l = mid+1;
            else r=mid-1;
        }
        return false;
    }
}

This answer time limit exceeds (when input 2^31-1). but if I changed l,r,mid to long (not int), it works. Why? Thanks