Sync LeetCode submission Runtime - 41 ms (59.93%), Memory - 28.1 MB (30.15%)

Author: jimit105

0398-random-pick-index/README.md

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+<p>Given an integer array <code>nums</code> with possible <strong>duplicates</strong>, randomly output the index of a given <code>target</code> number. You can assume that the given target number must exist in the array.</p>
+
+<p>Implement the <code>Solution</code> class:</p>
+
+<ul>
+	<li><code>Solution(int[] nums)</code> Initializes the object with the array <code>nums</code>.</li>
+	<li><code>int pick(int target)</code> Picks a random index <code>i</code> from <code>nums</code> where <code>nums[i] == target</code>. If there are multiple valid i&#39;s, then each index should have an equal probability of returning.</li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input</strong>
+[&quot;Solution&quot;, &quot;pick&quot;, &quot;pick&quot;, &quot;pick&quot;]
+[[[1, 2, 3, 3, 3]], [3], [1], [3]]
+<strong>Output</strong>
+[null, 4, 0, 2]
+
+<strong>Explanation</strong>
+Solution solution = new Solution([1, 2, 3, 3, 3]);
+solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
+solution.pick(1); // It should return 0. Since in the array only nums[0] is equal to 1.
+solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= nums.length &lt;= 2 * 10<sup>4</sup></code></li>
+	<li><code>-2<sup>31</sup> &lt;= nums[i] &lt;= 2<sup>31</sup> - 1</code></li>
+	<li><code>target</code> is an integer from <code>nums</code>.</li>
+	<li>At most <code>10<sup>4</sup></code> calls will be made to <code>pick</code>.</li>
+</ul>

0398-random-pick-index/solution.py

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+# Approach 3: Reservoir Sampling
+
+# Time: O(1) for pick(), O(n) for constructor
+# Space: O(n)
+
+import random
+
+class Solution:
+
+    def __init__(self, nums: List[int]):
+        self.indices = {} # Dictionary to store number -> list of indices
+        for i, num in enumerate(nums):
+            if num not in self.indices:
+                self.indices[num] = []
+            self.indices[num].append(i)
+        
+
+    def pick(self, target: int) -> int:
+        indices = self.indices[target]
+        return indices[random.randint(0, len(indices) - 1)]
+        
+
+
+# Your Solution object will be instantiated and called as such:
+# obj = Solution(nums)
+# param_1 = obj.pick(target)