題目：
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.
Example 1:
Input: num = "123", target = 6
Output: ["1+2+3", "1*2*3"]
Example 2:
Input: num = "232", target = 8
Output: ["2*3+2", "2+3*2"]
Example 3:
Input: num = "105", target = 5
Output: ["1*0+5","10-5"]
Example 4:
Input: num = "00", target = 0
Output: ["0+0", "0-0", "0*0"]
Example 5:
Input: num = "3456237490", target = 9191
Output: []
代碼：
class Solution {
public:
vector<string> addOperators(string num, int target) {
vector<string> res;
helper(num, target, 0, 0, "", res);
return res;
}
void helper(string num, int target, long diff, long curNum, string out, vector<string>& res) {
if (num.size() == 0 && curNum == target) {
res.push_back(out); return;
}
for (int i = 1; i <= num.size(); ++i) {
string cur = num.substr(0, i);
if (cur.size() > 1 && cur[0] == '0') return;
string next = num.substr(i);
if (out.size() > 0) {
helper(next, target, stoll(cur), curNum + stoll(cur), out + "+" + cur, res);
helper(next, target, -stoll(cur), curNum - stoll(cur), out + "-" + cur, res);
helper(next, target, diff * stoll(cur), (curNum - diff) + diff * stoll(cur), out + "*" + cur, res);
} else {
helper(next, target, stoll(cur), stoll(cur), cur, res);
}
}
}
};
思路：
巧妙的遞歸，想清楚串中加法和乘法的關係