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Commit d3038a8

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Add solution for Product of the Last K Numbers
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‎README.md

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- Longest Substring Without Repeating Characters | [Problem](https://leetcode.com/problems/longest-substring-without-repeating-characters) | [Solution](src/solutions/LongestSubstringWithoutRepeatingCharacters.java)
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- Sliding Window Maximum | [Problem](https://leetcode.com/problems/sliding-window-maximum) | [Solution](src/solutions/SlidingWindowMaximum.java)
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### Prefix Sum
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- Product of the Last K Numbers | [Problem](https://leetcode.com/problems/product-of-the-last-k-numbers) | [Solution](src/solutions/ProductOfNumbers.java)
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### Recursion
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- Closest Binary Search Tree Value | [Problem](https://leetcode.com/problems/closest-binary-search-tree-value)
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- Subtree Of Another Tree | [Problem](https://leetcode.com/problems/subtree-of-another-tree) | [Solution](src/solutions/SubtreeOfAnotherTree.java)

‎src/solutions/ProductOfNumbers.java

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package solutions;
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// [Problem] https://leetcode.com/problems/product-of-the-last-k-numbers
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import java.util.ArrayList;
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import java.util.List;
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// Prefix product
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class ProductOfNumbers {
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List<Integer> prefix; // O(n) space
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public ProductOfNumbers() {
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prefix = new ArrayList<>();
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prefix.add(1);
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}
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// O(1) time
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public void add(int num) {
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if (num != 0) {
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int lastPrefix = prefix.get(prefix.size() - 1);
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prefix.add(lastPrefix * num);
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} else {
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prefix = new ArrayList<>();
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prefix.add(1);
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}
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}
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// O(1) time
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public int getProduct(int k) {
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int n = prefix.size();
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if (k >= n) {
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return 0;
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}
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return prefix.get(n - 1) / prefix.get(n - k - 1);
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}
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// Test
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public static void main(String[] args) {
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ProductOfNumbers productOfNumbers = new ProductOfNumbers();
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productOfNumbers.add(3); // [3]
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productOfNumbers.add(0); // [3,0]
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productOfNumbers.add(2); // [3,0,2]
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productOfNumbers.add(5); // [3,0,2,5]
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productOfNumbers.add(4); // [3,0,2,5,4]
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System.out.println(productOfNumbers.getProduct(2)); // return 20. The product of the last 2 numbers is 5 * 4 = 20
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System.out.println(productOfNumbers.getProduct(3)); // return 40. The product of the last 3 numbers is 2 * 5 * 4 = 40
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System.out.println(productOfNumbers.getProduct(4)); // return 0. The product of the last 4 numbers is 0 * 2 * 5 * 4 = 0
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productOfNumbers.add(8); // [3,0,2,5,4,8]
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System.out.println(productOfNumbers.getProduct(2)); // return 32. The product of the last 2 numbers is 4 * 8 = 32
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}
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}

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