Approach 1: Ad-Hoc


If we see say S[0] == 'I', we can always put 0 as the first element; similarly, if we see S[0] == 'D', we can always put N as the first element.

Say we have a match for the rest of the string S[1], S[2], ... using N distinct elements. Notice it doesn't matter what the elements are, only that they are distinct and totally ordered. Then, putting 0 or N at the first character will match, and the rest of the elements (1, 2, ..., N or 0, 1, ..., N-1) can use the matching we have.


Keep track of the smallest and largest element we haven't placed. If we see an 'I', place the small element; otherwise place the large element.

Complexity Analysis

  • Time Complexity: , where is the length of S.

  • Space Complexity: .

Analysis written by: @awice.