This problem is equivalent to 1 1, 2 2, 3, ..., 100 100. These 5050 numbers are arranged from small to large. Find the middle number, that is, the number ranked 2525th.
Suppose this number is n,
Because n is preceded by at least1+2+...+n-1= n (n-1)/2 digits.
So n (n- 1)/2 < 2525.
Which one is n? -n-5050 & lt； 0
The solution is n