add: WeightedRandomSelection

Author: Jer3myYu

java/Binary Search/WeightedRandomSelection.java

@@ -0,0 +1,29 @@
+public class WeightedRandomSelection {
+    int[] prefixSum;
+
+    public WeightedRandomSelection(int[] weights) {
+        this.prefixSum = new int[weights.length];
+        for (int i = 0; i < weights.length; i++) {
+            prefixSum[i] = i == 0 ? weights[i] : prefixSum[i-1] + weights[i];
+        }
+    }
+
+    private int select() {
+        // Pick a random target between 1 and the largest endpoint on the number 
+        // line.
+        int target = (int)(Math.random() * prefixSum[prefixSum.length - 1]) + 1;
+        int left = 0;
+        int right = prefixSum.length - 1;
+        // Perform lower-bound binary search to find which endpoint (i.e., prefix 
+        // sum value) corresponds to the target.
+        while (left < right) {
+            int mid = (left + right) / 2;
+            if (prefixSum[mid] < target) {
+                left = mid + 1;
+            } else {
+                right = mid;
+            }
+        }
+        return left;
+    }
+}