1003. Check If Word Is Valid After Substitutions - notruilin/LeetCode GitHub Wiki
index = S.find("abc")
If there is no "abc" in S, index == -1
index = S.index("abc")
If there is no "abc" in S, it will throw an exception
Another solution is stack, pop "a" and "b" when each time we meet "c" If the top of the stack aren't "b" and "a", return False