add 528

Author: fishercoder1534

README.md

@@ -820,6 +820,7 @@ _If you like this project, please leave me a star._ ★
 |531|[Lonely Pixel I](https://leetcode.com/problems/lonely-pixel-i/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_531.java) |  |Medium | 
 |530|[Minimum Absolute Difference in BST](https://leetcode.com/problems/minimum-absolute-difference-in-bst/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_530.java) | |Easy| DFS
 |529|[Minesweeper](https://leetcode.com/problems/minesweeper/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_529.java) | |Medium | BFS
+|528|[Random Pick with Weight](https://leetcode.com/problems/random-pick-with-weight/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_528.java) | |Medium | Math, Binary Search, Prefix Sum, Randomized
 |527|[Word Abbreviation](https://leetcode.com/problems/word-abbreviation/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_527.java) | |Hard | 
 |526|[Beautiful Arrangement](https://leetcode.com/problems/beautiful-arrangement/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_526.java) | |Medium | Backtracking
 |525|[Contiguous Array](https://leetcode.com/problems/contiguous-array/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_525.java) |  |Medium | HashMap

src/main/java/com/fishercoder/solutions/_528.java

@@ -0,0 +1,35 @@
+package com.fishercoder.solutions;
+
+import java.util.Random;
+
+public class _528 {
+    public static class Solution1 {
+        Random random;
+        int[] preSums;
+
+        public Solution1(int[] w) {
+            this.random = new Random();
+            for (int i = 1; i < w.length; ++i) {
+                w[i] += w[i - 1];
+            }
+            this.preSums = w;
+        }
+
+        public int pickIndex() {
+            int len = preSums.length;
+            int idx = random.nextInt(preSums[len - 1]) + 1;
+            int left = 0, right = len - 1;
+            // search position
+            while (left < right) {
+                int mid = left + (right - left) / 2;
+                if (preSums[mid] == idx)
+                    return mid;
+                else if (preSums[mid] < idx)
+                    left = mid + 1;
+                else
+                    right = mid;
+            }
+            return left;
+        }
+    }
+}

src/test/java/com/fishercoder/_528Test.java

@@ -0,0 +1,29 @@
+package com.fishercoder;
+
+import com.fishercoder.solutions._528;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class _528Test {
+    private static _528.Solution1 solution1;
+    private static int expected;
+
+    @Test
+    public void test1() {
+        solution1 = new _528.Solution1(new int[]{1});
+        expected = 0;
+        assertEquals(expected, solution1.pickIndex());
+    }
+
+    @Test
+    public void test2() {
+        solution1 = new _528.Solution1(new int[]{1, 3});
+        System.out.println(solution1.pickIndex());
+        System.out.println(solution1.pickIndex());
+        System.out.println(solution1.pickIndex());
+        System.out.println(solution1.pickIndex());
+        System.out.println(solution1.pickIndex());
+    }
+
+}