LeetCode-in-Java

282. Expression Add Operators

Hard

Given a string num that contains only digits and an integer target, return all possibilities to insert the binary operators '+', '-', and/or '*' between the digits of num so that the resultant expression evaluates to the target value.

Note that operands in the returned expressions should not contain leading zeros.

Example 1:

Input: num = “123”, target = 6

Output: [“1*2*3”,”1+2+3”]

Explanation: Both “1*2*3” and “1+2+3” evaluate to 6.

Example 2:

Input: num = “232”, target = 8

Output: [“2*3+2”,”2+3*2”]

Explanation: Both “2*3+2” and “2+3*2” evaluate to 8.

Example 3:

Input: num = “105”, target = 5

Output: [“1*0+5”,”10-5”]

Explanation:

Both "1*0+5" and "10-5" evaluate to 5.
Note that "1-05" is not a valid expression because the 5 has a leading zero. 

Example 4:

Input: num = “00”, target = 0

Output: [“0*0”,”0+0”,”0-0”]

Explanation:

"0*0", "0+0", and "0-0" all evaluate to 0.
Note that "00" is not a valid expression because the 0 has a leading zero. 

Example 5:

Input: num = “3456237490”, target = 9191

Output: []

Explanation: There are no expressions that can be created from “3456237490” to evaluate to 9191.

Constraints:

• 1 <= num.length <= 10
• num consists of only digits.
• -231 <= target <= 231 - 1

Solution

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

@SuppressWarnings("java:S107")
public class Solution {
    public List<String> addOperators(String num, int target) {
        List<String> res = new ArrayList<>();
        if (num.isEmpty() || Long.parseLong(num) > Integer.MAX_VALUE) {
            return res;
        }
        char[] list = num.toCharArray();
        dfs(res, list, 0, 0, target, new char[2 * list.length - 1], 0, 1, '+', 0);
        return res;
    }

    private void dfs(
            List<String> res,
            char[] list,
            int i,
            int j,
            int target,
            char[] arr,
            int val,
            int mul,
            char preOp,
            int join) {
        arr[j++] = list[i];
        int digit = 10 * join + (list[i] - '0');
        int result = val + mul * digit;
        if (preOp == '-') {
            result = val - mul * digit;
        }
        if (i == list.length - 1) {
            if (result == target) {
                StringBuilder sb = new StringBuilder();
                for (int k = 0; k < j; k++) {
                    sb.append(arr[k]);
                }
                res.add(sb.toString());
            }
            return;
        }
        if (preOp == '+') {
            arr[j] = '+';
            dfs(res, list, i + 1, j + 1, target, arr, val + mul * digit, 1, '+', 0);
            arr[j] = '-';
            dfs(res, list, i + 1, j + 1, target, arr, val + mul * digit, 1, '-', 0);
            arr[j] = '*';
            dfs(res, list, i + 1, j + 1, target, arr, val, mul * digit, '+', 0);
            if (digit != 0) {
                dfs(res, list, i + 1, j, target, arr, val, mul, '+', digit);
            }
        } else {
            arr[j] = '+';
            dfs(res, list, i + 1, j + 1, target, arr, val - mul * digit, 1, '+', 0);
            arr[j] = '-';
            dfs(res, list, i + 1, j + 1, target, arr, val - mul * digit, 1, '-', 0);
            arr[j] = '*';
            dfs(res, list, i + 1, j + 1, target, arr, val, mul * digit, '-', 0);
            if (digit != 0) {
                dfs(res, list, i + 1, j, target, arr, val, mul, '-', digit);
            }
        }
    }
}

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