1 class Solution {
2 List<String> res = new ArrayList<>();
3 public List<String> addOperators(String num, int target) {
4 if(num.length() == 0) return res;
5 backtrack(num, target, "", 0, 0, 0);
6 return res;
7 }
8
9 public void backtrack(String num, int target, String str, int pos, long total, long prev){
10 if(pos == num.length()){
11 if(target== total){
12 res.add(str);
13 // System.out.println(total);
14 }
15
16 return;
17 }
18 for(int i = pos; i < num.length(); i++){
19 if(i != pos && num.charAt(pos) == '0') break; // i!=pos
20 long cur = Long.parseLong(num.substring(pos, i+1));
21 if(pos == 0){
22 backtrack(num, target, str+cur, i+1, cur, cur);
23 }else{
24 backtrack(num, target, str + "+" + cur, i+1, total+cur, cur);
25 backtrack(num, target, str + "-" + cur, i+1, total-cur, -cur); //是i+1 是-cur
26 backtrack(num, target, str + "*" + cur, i+1, total - prev + prev * cur, prev*cur);
27 }
28 }
29 }
30 }
