282. 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 binary operators (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> res = new ArrayList<String>();        if(num == null || num.length() == 0) return res;        helper(num, target, res, "", 0, 0, 0);        return res;    }    public void helper(String num, int target, List<String> res, String path, int pos, long eval, long mul) {        if (pos == num.length()) {            if (eval == target) {                res.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(num, target, res, path+cur, i+1, cur, cur);            else {                helper(num, target, res, path+"+"+cur, i+1, eval+cur, cur);                helper(num, target, res, path+"-"+cur, i+1, eval-cur, -cur);                helper(num, target, res, path+"*"+cur, i+1, eval-mul+mul*cur, cur*mul);            }        }    }}

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