def addOperators(self, num, target):
        ret = []
        self.dfs(num, target, 0, "", 0, 0, ret)
        return ret
Adapted from https://leetcode.com/discuss/58614/java-standard-backtrace-ac-solutoin-short-and-clear
dataset/ETHPy150Open algorhythms/LeetCode/282 Expression Add Operators.py/Solution.addOperators