1. We will use backtracking to generate all possible combinations of the input string by inserting the operators '+', '-', and '*' between the digits. 2. We will define a helper function that takes ...

/// like Earley parser, O(4^n) (string concatenation treated as O(1)) forward(k+1, s+a[k], add, mul, last*10+x); forward(k+1, s+'*'+a[k], add, mul*last, x); forward ...