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Commit a4c50bb

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Merge pull request #323 from cheehwatang/add-923-3SumWithMultiplicity
Add 923. 3Sum With Multiplicity (Counting)
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‎README.md‎

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<th>Solution</th>
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<th>Topics</th>
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</tr>
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<tr>
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<td align="center">September 2nd</td>
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<td>923. <a href="https://leetcode.com/problems/3sum-with-multiplicity/">3Sum With Multiplicity</a></td>
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<td align="center">$\text{\color{Dandelion}Medium}$</td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/923.%203Sum%20With%20Multiplicity/ThreeSumWithMultiplicity_Counting.java">Counting</a>
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</td>
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<td align="center">
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<a href="#array">Array</a>,
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<a href="#counting">Counting</a>
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</td>
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</tr>
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<tr>
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<td align="center">September 1st</td>
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<td>16. <a href="https://leetcode.com/problems/3sum-closest/">3Sum Closest</a></td>
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<a href="#array">Array</a>
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</td>
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</tr>
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<tr>
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<td align="center">August 28th</td>
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<td>3. <a href="https://leetcode.com/problems/longest-substring-without-repeating-characters/">Longest Substring Without Repeating Characters</a></td>
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<td align="center">$\text{\color{Dandelion}Medium}$</td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/3.%20Longest%20Substring%20Without%20Repeating%20Characters/LongestSubstringWithoutRepeatingCharacters_HashTable.java">Hash Table & Sliding Window</a>
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</td>
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<td align="center">
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<a href="#hash-table">Hash Table</a>,
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<a href="#sliding-window">Sliding Window</a>,
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<a href="#string">String</a>
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</td>
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</tr>
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</table>
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</br>
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<hr>
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</td>
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<td></td>
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</tr>
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<tr>
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<td align="center">923</td>
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<td><a href="https://leetcode.com/problems/3sum-with-multiplicity/">3Sum With Multiplicity</a></td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/923.%203Sum%20With%20Multiplicity/ThreeSumWithMultiplicity_Counting.java">Java</a>
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</td>
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<td align="center">$\text{\color{Dandelion}Medium}$</td>
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<td align="center">
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<a href="#array">Array</a>,
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<a href="#counting">Counting</a>
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</td>
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<td></td>
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</tr>
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<tr>
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<td align="center">929</td>
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<td><a href="https://leetcode.com/problems/unique-email-addresses/">Unique Email Addresses</a></td>
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/383.%20Ransom%20Note/RansomNote_HashTable.java"><em>Hash Table</em></a>
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</td>
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</tr>
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<tr>
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<td align="center">923</td>
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<td><a href="https://leetcode.com/problems/3sum-with-multiplicity/">3Sum With Multiplicity</a></td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/923.%203Sum%20With%20Multiplicity/ThreeSumWithMultiplicity_Counting.java">Java</a>
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</td>
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<td align="center">$\text{\color{Dandelion}Medium}$</td>
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<td align="center">
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<a href="#array">Array</a>,
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<a href="#counting">Counting</a>
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</td>
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<td></td>
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</tr>
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<tr>
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<td align="center">1497</td>
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<td><a href="https://leetcode.com/problems/check-if-array-pairs-are-divisible-by-k/">Check If Array Pairs Are Divisible by k</a></td>
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package com.cheehwatang.leetcode;
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// Time Complexity : O(n),
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// where 'n' is the length of 'arr'.
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// We traverse 'arr' once to count the frequency of the elements.
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//
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// Space Complexity : O(1),
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// as the auxiliary space used is independent of the input 'arr',
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// with the counting array with size of 101 regardless of the input 'arr'.
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public class ThreeSumWithMultiplicity_Counting {
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// Approach:
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// Using a counting array, get the frequency of all the integers,
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// then check all the possible combinations to get the sum.
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// Note that, i < j < k and arr[i] + arr[j] + arr[k] == target,
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// do not require the index as the sum is not dependent on the index.
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public int threeSumMulti(int[] arr, int target) {
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// With the constraint of 0 <= arr[i] <= 100, count the frequency of all the integers/
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int[] counting = new int[101];
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for (int integer : arr) counting[integer]++;
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// Keep the count as long type, so as only need to perform the modulo once at the end.
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long count = 0;
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// Traverse the counting array in two for-loops to get the value for 'i' and 'j'.
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for (int i = 0; i <= 100; i++) {
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for (int j = i; j <= 100; j++) {
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// Get the k integer to check if it is in the counting array.
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int k = target - i - j;
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if (k < 0 || k > 100) continue;
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if (counting[k] > 0) {
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// If the triplet is of the same integers (eg. [1,1,1], target = 3),
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// then perform geometric sum for 3 elements.
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if (i == k && j == k)
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count += (long) counting[i] * (counting[i] - 1) * (counting[i] - 2) / 6;
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// If i and j is identical, but different from k (eg. [1,1,2], target = 4),
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// then perform geometric sum for i & j, then multiply with k.
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else if (i == j)
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count += (long) counting[i] * (counting[i] - 1) / 2 * counting[k];
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// If both i and j is smaller than k, then get the multiples of all the frequency.
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// This is to make sure no duplicates of results
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// (eg, [1,2,3], target = 6), only [1,2,3] is counted, but not [2,1,3] or [3,2,1], etc.)
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else if (i < k && j < k)
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count += (long) counting[i] * counting[j] * counting[k];
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}
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}
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}
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return (int) (count % (1e9 + 7));
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}
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}

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