/**

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

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

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

helper(res, "", num, target, 0, 0, 0);

return res;

}

private void helper(List<String> res, String path, String num, int target, int pos, long val, long pre) {

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

if (target == val) {

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(res, path + cur, num, target, i + 1, cur, cur);

} else {

helper(res, path + "+" + cur, num, target, i + 1, val + cur, cur);

helper(res, path + "-" + cur, num, target, i + 1, val - cur, -cur);

helper(res, path + "*" + cur, num, target, i + 1, val - pre + pre * cur, pre * cur);

}

}

}