public List<String> addOperators(String num, int target) {

List<String> result = new ArrayList<>();

if (num == null || num.length() == 0) return result;

addOperators(num, 0, target, 0, 0, "", result);

return result;

}

private void addOperators(String num, int start, int target, long sum,

long preNum, String curResult, List<String> result) {

if (start == num.length()) {

if (sum == target) result.add(curResult);

return;

}

for (int i = start; i < num.length(); i++) {

String curStr = num.substring(start, i + 1);

if (curStr.length() > 1 && curStr.charAt(0) == '0') {

break;

}

long curNum = Long.parseLong(curStr);

if (curResult.isEmpty()) {

addOperators(num, i + 1, target, curNum, curNum, curStr, result);

} else {

addOperators(num, i + 1, target, sum + curNum, curNum, curResult + "+" + curNum, result);

addOperators(num, i + 1, target, sum - curNum, -curNum, curResult + "-" + curNum, result);

addOperators(num, i + 1, target, preNum * curNum + sum - preNum, preNum * curNum, curResult + "*" + curNum, result);

}

}

}

}