LeetCode [282] Expression Add Operators

282. Expression Add Operators

Hard

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: []

 

Constraints:

• 0 <= num.length <= 10
• num only contain digits.

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typedef long long ll; class Solution { public: vector<string> addOperators(string num, int target) { vector<string> ret; dfs(num, "", 0, 0, 0, ret, target); return ret; } void dfs(string num, string exp, int pos, ll curVal, ll preVal, vector<string>& ret, ll target) { if(pos == num.size() && curVal==target) { ret.push_back(exp); } else if(pos<num.size()) { for(int i=pos; i<num.size(); ++i) { string v = num.substr(pos, i-pos+1); if(v.size()>1 && v[0]=='0') break; if(pos==0) { dfs(num, v, i+1, stoll(v), stoll(v), ret, target); } else { dfs(num, exp+"+"+v, i+1, curVal+stoll(v), stoll(v), ret, target); dfs(num, exp+"-"+v, i+1, curVal-stoll(v), -stoll(v), ret, target); dfs(num, exp+"*"+v, i+1, curVal-preVal+preVal*stoll(v), preVal*stoll(v), ret, target); } } } } };