add Step3

Author: colorbox

278/step3.cpp

@@ -0,0 +1,17 @@
+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 - 1;
+            } else {
+                left = mid + 1;
+            }
+        }
+        return left;
+    }
+};