Binary Search Example

Author: rachitiitr

LeetCode/878.nth-magical-number.cpp

@@ -0,0 +1,21 @@
+int countMultiples(long long x, long long a, long long b) {
+    // get # of multiples of a or b in range [1, x]
+    long long lcm = a * b / __gcd(a, b);
+    return x/a + x/b - x/lcm;
+}
+
+class Solution {
+public:
+    int nthMagicalNumber(int n, int a, int b) {
+        // a = 2, b = 3 in our example
+        long long lo = 1, hi = min(a, b) * 1LL * n, mod = 1e9 + 7;
+        while(lo < hi) {
+            long long mid = lo + (hi - lo) / 2;
+            if (countMultiples(mid, a, b) < n)
+                lo = mid + 1;
+            else
+                hi = mid;
+        }
+        return hi % mod;
+    }
+};