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

My Javascript implementation:
Will improve the code using web worker later on

// was not allowed to use eval , so this is my replacement for the eval function.
function evaluate(expr) {
  return new Function('return '+expr)();
 }
 function calc(expr,input,target) { 
   if (input.length==1) { 
    // I'm not allowed to use eval, so I will use my function evaluate
    //if (eval(expr+input)==target) console.log(expr+input+"="+target);
    if (evaluate(expr+input)==target) document.body.innerHTML+=expr+input+"="+target+"
";
   }
   else { 
     for(var i=1;i<=input.length;i++) {
      var left=input.substring(0,i);
      var right=input.substring(i);
      ['+','-','*','/'].forEach(function(oper) {
         calc(expr+left+oper,right,target);
      },this);
     }
   }
 };
 function f(input,total) {
  calc("",input,total);
 }