Add step5 with bisect_left

https://github.com/python/cpython/blob/main/Lib/bisect.py

Author: colorbox

278/step5.cpp

@@ -0,0 +1,23 @@
+/*
+bisect_left
+https://github.com/python/cpython/blob/main/Lib/bisect.py
+を参考に書いたコード
+*/
+class Solution {
+public:
+    int firstBadVersion(int n) {
+        int left = 1;
+        int right = n;
+
+        while (left < right) {
+            int mid = left + (right - left) / 2;
+            if (isBadVersion(mid)) {
+                right = mid;
+            } else {
+                left = mid + 1;
+            }
+        }
+
+        return left;
+    }
+};