Generate all combinations of mathematical expressions that add to target (Java homework/interview)

Question

I\'ve tried to solve the problem below for a coding challenge but could not finish it in 1 hour. I have an idea on how the algorithm works but I\'m not quite sure how to best im

Answer 1

I don't think it's necessary to build a tree, you should be able to calculate as you go -- you just need to delay additions and subtractions slightly in order to be able take the precedence into account correctly:

static void check(double sum, double previous, String digits, double target, String expr) {
   if (digits.length() == 0) {
     if (sum + previous == target) {
       System.out.println(expr + " = " + target);
     }
   } else {
     for (int i = 1; i <= digits.length(); i++) {
       double current = Double.parseDouble(digits.substring(0, i));
       String remaining = digits.substring(i);
       check(sum + previous, current, remaining, target, expr + " + " + current);
       check(sum, previous * current, remaining, target, expr + " * " + current);
       check(sum, previous / current, remaining, target, expr + " / " + current);
       check(sum + previous, -current, remaining, target, expr + " - " + current);
     }
   }
 }

 static void f(String digits, double target) {
   for (int i = 1; i <= digits.length(); i++) {
     String current = digits.substring(0, i);
     check(0, Double.parseDouble(current), digits.substring(i), target, current);
   }
 }