Add step4 as simple logic

Author: colorbox

278/step4.cpp

@@ -0,0 +1,21 @@
+class Solution {
+public:
+    int firstBadVersion(int n) {
+        if (isBadVersion(1)) {
+            return 1;
+        }
+
+        int left = 1;
+        int right = n;
+
+        while (left + 1 < right) {
+            int mid = left + (right - left) / 2;
+            if (isBadVersion(mid)) {
+                right = mid;
+            } else {
+                left = mid;
+            }
+        }
+        return right;
+    }
+};