Expression Add Operators

Source

• leetcode: Expression Add Operators

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 -> []

Java

public class Solution {
    public List<String> addOperators(String num, int target) {
        List<String> rst = new ArrayList<String>();
        if(num == null || num.length() == 0) return rst;
        helper(rst, "", num, target, 0, 0, 0);
        return rst;
    }
    public void helper(List<String> rst, String path, String num, int target, int pos, long curRes, long preNum){
        if(pos == num.length()){
            if(target == curRes)
                rst.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(rst, path+cur, num, target, i+1, cur, cur);
            }
            else{
                helper(rst, path+"+"+cur, num, target, i+1, curRes+cur , cur);

                helper(rst, path+"-"+cur, num, target, i+1, curRes-cur, -cur);

                helper(rst, path+"*"+cur, num, target, i+1, (curRes-preNum)+preNum*cur, preNum*cur);
            }
        }
    }
}

Reference

Expression Add Operators 添加运算符