Expression Add Operators

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Given a string that contains only digits 0-9 and a target value, return all possibilities to add binaryoperators (not unary) +, -, or * between the digits so they evaluate to the target value.

Examples:

"123", 6 -> ["1+2+3", "1*2*3"] "232", 8 -> ["2*3+2", "2+3*2"]"105", 5 -> ["1*0+5","10-5"]"00", 0 -> ["0+0", "0-0", "0*0"]"3456237490", 9191 -> []

public class Solution {    public List<String> addOperators(String num, int target) {        List<String> rst = new ArrayList<String>();        if(num == null || num.length() == 0) return rst;        helper(rst, "", num, target, 0, 0, 0);        return rst;    }    public void helper(List<String> rst, String path, String num, int target, int pos, long eval, long multed){        if(pos == num.length()){            if(target == eval)                rst.add(path);            return;        }        for(int i = pos; i < num.length(); i++){            if(i != pos && num.charAt(pos) == '0') break;            long cur = Long.parseLong(num.substring(pos, i + 1));            if(pos == 0){                helper(rst, path + cur, num, target, i + 1, cur, cur);            }            else{                helper(rst, path + "+" + cur, num, target, i + 1, eval + cur , cur);                helper(rst, path + "-" + cur, num, target, i + 1, eval -cur, -cur);                helper(rst, path + "*" + cur, num, target, i + 1, eval - multed + multed * cur, multed * cur );            }        }    }}

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