vault backup: 2022-07-09 10:15:12

OJ notes/pages/Leetcode First-Bad-Version.md

@@ -61,10 +61,45 @@ Note that [[Leetcode First-Bad-Version#Constraints]], n can be 2**31, which mean
 
 To address that, according to [[Binary Search Algorithm#How to implement Binary search]], use `mid = l + (r - l) / 2`
 
+In my first iteration, 
 I use a `first` variable to keep track of the first bad version.
 
+Later I realized that by the definition of Bi-search, left boundary will converge to the first one.
+
 ### Solution
 
+Second version, 0ms
+```cpp
+// The API isBadVersion is defined for you.
+// bool isBadVersion(int version);
+
+class Solution {
+public:
+  int firstBadVersion(int n) {
+    // variant of BS
+    // 1-indexed
+    
+    int r = n;
+    int l = 1;
+    int mid;
+    
+    do {
+      mid = l + (r - l) / 2;
+        
+      if (isBadVersion(mid)) {
+        // Search left
+        r = mid - 1;
+      } else {
+        l = mid + 1;
+      }
+    } while (l <= r);
+      
+    return l;
+  }
+};
+```
+
+First iteration, 4ms
 ```cpp
 // The API isBadVersion is defined for you.
 // bool isBadVersion(int version);