_282.java

package com.fishercoder.solutions.firstthousand;

import java.util.ArrayList;
import java.util.List;

public class _282 {

    public static class Solution1 {
        public List<String> addOperators(String num, int target) {
            List<String> res = new ArrayList<>();
            StringBuilder sb = new StringBuilder();
            dfs(res, sb, num, 0, target, 0, 0);
            return res;
        }

        private void dfs(
                List<String> res,
                StringBuilder sb,
                String num,
                int pos,
                int target,
                long prev,
                long multi) {
            if (pos == num.length()) {
                if (target == prev) {
                    res.add(sb.toString());
                }
                return;
            }
            for (int i = pos; i < num.length(); i++) {
                if (num.charAt(pos) == '0' && i != pos) {
                    break;
                }
                long curr = Long.parseLong(num.substring(pos, i + 1));
                int len = sb.length();
                if (pos == 0) {
                    dfs(res, sb.append(curr), num, i + 1, target, curr, curr);
                    sb.setLength(len);
                } else {
                    dfs(res, sb.append("+").append(curr), num, i + 1, target, prev + curr, curr);
                    sb.setLength(len);

                    dfs(res, sb.append("-").append(curr), num, i + 1, target, prev - curr, -curr);
                    sb.setLength(len);

                    dfs(
                            res,
                            sb.append("*").append(curr),
                            num,
                            i + 1,
                            target,
                            prev - multi + multi * curr,
                            multi * curr);
                    sb.setLength(len);
                }
            }
        }
    }
}