[Search] Add a solution to Random Pick with Weight

README.md

@@ -4,7 +4,7 @@
 ![Leetcode](./logo.png?style=centerme)
 
 ## Progress
-[Problem Status](#problem-status) shows the latest progress to all 1000+ questions. Currently we have 314 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them.
+[Problem Status](#problem-status) shows the latest progress to all 1000+ questions. Currently we have 315 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them.
 
 ## Contributors
 
@@ -356,6 +356,7 @@
 [Search Insert Position](https://leetcode.com/problems/search-insert-position/)| [Swift](./Search/SearchForARange.swift)| Medium| O(logn)| O(1)|
 [Peak Index in a Mountain Array](https://leetcode.com/problems/peak-index-in-a-mountain-array/)| [Swift](./Search/PeakIndexMountainArray.swift)| Easy| O(logn)| O(1)|
 [Find Peak Element](https://leetcode.com/problems/find-peak-element/)| [Swift](./Search/FindPeakElement.swift)| Medium| O(logn)| O(1)|
+[Random Pick with Weight](https://leetcode.com/problems/random-pick-with-weight/)| [Swift](./Search/RandomPickWeight.swift)| Medium| O(logn)| O(1)|
 [Sqrt(x)](https://leetcode.com/problems/sqrtx/)| [Swift](./Search/Sqrtx.swift)| Medium| O(logn)| O(1)|
 [Median of Two Sorted Arrays](https://leetcode.com/problems/median-of-two-sorted-arrays/)| [Swift](./Search/MedianTwoSortedArrays.swift)| Hard| O(log(m + n))| O(1)|
 [Minimize Max Distance to Gas Station](https://leetcode.com/problems/minimize-max-distance-to-gas-station/)| [Swift](./Search/MinimizeMaxDistanceGasStation.swift)| Hard| O(nlogm)| O(1)|

Search/RandomPickWeight.swift

@@ -0,0 +1,50 @@
+/**
+ * Question Link: https://leetcode.com/problems/random-pick-with-weight/
+ * Primary idea: Random select a number from sum of the array, and search the first number 
+ *               greater than the number in sums array constructed from the original array.
+ *
+ * Time Complexity: O(logn), Space Complexity: O(n)
+ */
+
+
+class RandomPickWeight {
+
+    var sums: [Int] 
+
+    init(_ w: [Int]) {
+        sums = w
+
+        for i in 1..<w.count {
+            sums[i] += sums[i - 1] 
+        }
+    }
+
+    func pickIndex() -> Int {
+        guard let sum = sums.last else {
+            return -1
+        }
+
+        return findFirstGreaterThan(Int.random(in: 0..<sum))
+    }
+
+    private func findFirstGreaterThan(_ num: Int) -> Int {
+        var left = 0, right = sums.count - 1
+
+        while left < right {
+            let mid = (right - left) / 2 + left
+            if sums[mid] > num {
+                right = mid
+            } else {
+                left = mid + 1
+            }
+        }
+
+        return left
+    }
+}
+
+/**
+ * Your Solution object will be instantiated and called as such:
+ * let obj = Solution(w)
+ * let ret_1: Int = obj.pickIndex()
+ */