282. Expression Add Operators

class Solution {
    List<String> res = new ArrayList<>();
    public List<String> addOperators(String num, int target) {
        if(num.length() == 0) return res;
        backtrack(num, target, "", 0, 0, 0);
        return res;
    }
    
    public void backtrack(String num, int target, String str, int pos, long total, long prev){
        if(pos == num.length()){
            if(target== total){
                res.add(str);
                // System.out.println(total);
            }
            
            return;
        }
        for(int i = pos; i < num.length(); i++){
            if(i != pos && num.charAt(pos) == '0') break; // i!=pos
            long cur = Long.parseLong(num.substring(pos, i+1));
            if(pos == 0){
                backtrack(num, target, str+cur, i+1, cur, cur);
            }else{
                backtrack(num, target, str + "+" + cur, i+1, total+cur, cur);
                backtrack(num, target, str + "-" + cur, i+1, total-cur, -cur); //是i+1 是-cur
                backtrack(num, target, str + "*" + cur, i+1, total - prev + prev * cur, prev*cur);
            }
        }
    }
}