I suggest U 381E - Сережа и скобочки and 483D - Интересный массив

Firstly, practice on segment tree with single updates, here are some problems:

Enter here and solve these problems, they are very easy, for beginners (8 problems): https://www.e-olymp.com/en/contests/7919

Then try to solve these problems, they are not very hard but harder than previous one (21 problems, some problems can be same with previous one, some of them can be found in spoj too): https://www.e-olymp.com/en/contests/8374

Also, big archive of segment tree problems can be found here: https://a2oj.com/Category.jsp?ID=25

Good luck ;)