adding 3rd solution (#85)

LeetCode/Algorithms/Medium/RestoreIPAddresses.cpp

@@ -0,0 +1,32 @@
+class Solution {
+ public:
+  vector<string> restoreIpAddresses(const string& s) {
+    vector<string> ans;
+    dfs(s, 0, {}, ans);
+    return ans;
+  }
+
+ private:
+  void dfs(const string& s, int start, vector<string>&& path,
+           vector<string>& ans) {
+    if (path.size() == 4 && start == s.length()) {
+      ans.push_back(path[0] + "." + path[1] + "." + path[2] + "." + path[3]);
+      return;
+    }
+    if (path.size() == 4 || start == s.length())
+      return;
+
+    for (int length = 1; length <= 3; ++length) {
+      if (start + length > s.length())
+        return;  // Out of bound
+      if (length > 1 && s[start] == '0')
+        return;  // Leading '0'
+      const string& num = s.substr(start, length);
+      if (stoi(num) > 255)
+        return;
+      path.push_back(num);
+      dfs(s, start + length, move(path), ans);
+      path.pop_back();
+    }
+  }
+};