diff --git a/solved/LeetCode/Challenges/2020/January/07.cc b/solved/LeetCode/Challenges/2020/January/07.cc
new file mode 100644
index 00000000..d3ba3f8a
--- /dev/null
+++ b/solved/LeetCode/Challenges/2020/January/07.cc
@@ -0,0 +1,26 @@
+class Solution {
+public:
+  int lengthOfLongestSubstring(string s) {
+    map<char, int> freq;
+    int i = 0;
+    int j = 0;
+    int best = 0;
+    while (i < s.size() && j < s.size()) {
+      while (j < s.size() && freq[s[j]] < 1) {
+        freq[s[j]]++;
+        j++;
+        best = max(best, j - i);
+      }
+      if (j >= s.size()) {
+        break;
+      }
+      while (i < s.size() && s[i] != s[j]) {
+        freq[s[i]]--;
+        i++;
+      }
+      freq[s[i]]--;
+      i++;
+    }
+    return best;
+ }
+};