Merge pull request #324 from cheehwatang/add-923-3SumWithMultiplicity

Add 923. 3Sum With Multiplicity (Hash Table)

Author: cheehwatang

README.md

@@ -27,6 +27,18 @@
     <th>Solution</th>
     <th>Topics</th>
   </tr>
+  <tr>
+    <td align="center">September 3rd</td>
+    <td>923. <a href="https://leetcode.com/problems/3sum-with-multiplicity/">3Sum With Multiplicity</a></td>
+    <td align="center">$\text{\color{Dandelion}Medium}$</td>
+    <td align="center">
+      <a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/923.%203Sum%20With%20Multiplicity/ThreeSumWithMultiplicity_HashTable.java">Hash Table</a>
+    </td>
+    <td align="center">
+      <a href="#array">Array</a>,
+      <a href="#hash-table">Hash Table</a>
+    </td>
+  </tr>
   <tr>
     <td align="center">September 2nd</td>
     <td>923. <a href="https://leetcode.com/problems/3sum-with-multiplicity/">3Sum With Multiplicity</a></td>
@@ -954,13 +966,15 @@
   <tr>
     <td align="center">923</td>
     <td><a href="https://leetcode.com/problems/3sum-with-multiplicity/">3Sum With Multiplicity</a></td>
-    <td align="center">
-      <a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/923.%203Sum%20With%20Multiplicity/ThreeSumWithMultiplicity_Counting.java">Java</a>
+    <td align="center">Java with
+      <a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/923.%203Sum%20With%20Multiplicity/ThreeSumWithMultiplicity_Counting.java">Counting</a> or
+      <a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/923.%203Sum%20With%20Multiplicity/ThreeSumWithMultiplicity_HashTable.java">Hash Table</a>
     </td>
     <td align="center">$\text{\color{Dandelion}Medium}$</td>
     <td align="center">
       <a href="#array">Array</a>,
-      <a href="#counting">Counting</a>
+      <a href="#counting">Counting</a>,
+      <a href="#hash-table">Hash Table</a>
     </td>
     <td></td>
   </tr>
@@ -3291,9 +3305,12 @@
     <td align="center">$\text{\color{Dandelion}Medium}$</td>
     <td align="center">
       <a href="#array">Array</a>,
-      <a href="#counting">Counting</a>
+      <a href="#counting">Counting</a>,
+      <a href="#hash-table">Hash Table</a>
+    </td>
+    <td>Solution Using
+      <a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/923.%203Sum%20With%20Multiplicity/ThreeSumWithMultiplicity_HashTable.java"><em>Hash Table</em></a>
     </td>
-    <td></td>
   </tr>
   <tr>
     <td align="center">1497</td>
@@ -5241,6 +5258,22 @@
     </td>
     <td></td>
   </tr>
+  <tr>
+    <td align="center">923</td>
+    <td><a href="https://leetcode.com/problems/3sum-with-multiplicity/">3Sum With Multiplicity</a></td>
+    <td align="center">
+      <a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/923.%203Sum%20With%20Multiplicity/ThreeSumWithMultiplicity_HashTable.java">Java</a>
+    </td>
+    <td align="center">$\text{\color{Dandelion}Medium}$</td>
+    <td align="center">
+      <a href="#array">Array</a>,
+      <a href="#counting">Counting</a>,
+      <a href="#hash-table">Hash Table</a>
+    </td>
+    <td>Solution Using
+      <a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/923.%203Sum%20With%20Multiplicity/ThreeSumWithMultiplicity_Counting.java"><em>Counting</em></a>
+    </td>
+  </tr>
   <tr>
     <td align="center">929</td>
     <td><a href="https://leetcode.com/problems/unique-email-addresses/">Unique Email Addresses</a></td>

solutions/923. 3Sum With Multiplicity/ThreeSumWithMultiplicity_HashTable.java

@@ -0,0 +1,38 @@
+package com.cheehwatang.leetcode;
+
+import java.util.HashMap;
+import java.util.Map;
+
+// Time Complexity  : O(n^2),
+// where 'n' is the length of 'arr'.
+// We traverse 'arr' in a nested for-loop, checking each triplet for each 'arr[i]'.
+//
+// Space Complexity : O(n^2),
+// where 'n' is the length of 'arr'.
+// For each 'arr[i]', there are 'n' number of possible differences to store in the HashTable.
+
+public class ThreeSumWithMultiplicity_HashTable {
+
+    // Approach:
+    // Using a HashTable to record "target - arr[k]" of the equation, then traverse both 'i' and 'j' to check if equal.
+    // Note that we rearranged the equation as arr[i] + arr[j] == target - arr[k].
+    // Each loop recording the frequency of "target - arr[k]", add the frequency if equal.
+
+    public int threeSumMulti(int[] arr, int target) {
+        int n = arr.length;
+        // Store the first value of 'arr[k]' which is arr[n - 1], starting from right to left each iteration.
+        Map<Integer, Integer> map = new HashMap<>();
+        map.put(target - arr[n - 1], 1);
+        long count = 0;
+        for (int j = n - 2; j >= 0; j--) {
+            // Add the frequency of "target - arr[k]" to the count if found.
+            for (int i = j - 1; i >= 0; i--) {
+                count += map.getOrDefault(arr[i] + arr[j], 0);
+            }
+            // Once done with all the 'j' for this loop, increase the frequency for "target - arr[j]".
+            int difference = target - arr[j];
+            map.put(difference, map.getOrDefault(difference, 0) + 1);
+        }
+        return (int) (count % (1e9 + 7));
+    }
+}