Added 3sum solution

Author: nhistory

3sum/nhistory.js

@@ -0,0 +1,43 @@
+var threeSum = function (nums) {
+  // Sort nums array
+  const sortedNums = nums.sort((a, b) => a - b);
+  let result = [];
+
+  // Start iteration to pick first element for 3sum
+  for (let i = 0; i < sortedNums.length; i++) {
+    // Check if the first element is already greater than 0, no valid triplets possible after this point
+    if (sortedNums[i] > 0) {
+      break;
+    }
+
+    // Skip duplicates of the first element to avoid redundant triplets
+    if (i > 0 && sortedNums[i] === sortedNums[i - 1]) {
+      continue;
+    }
+
+    // Iterate to find sum of two pointer and nums[i]
+    let left = i + 1;
+    let right = sortedNums.length - 1;
+
+    while (left < right) {
+      let sum = sortedNums[i] + sortedNums[left] + sortedNums[right];
+
+      if (sum === 0) {
+        result.push([sortedNums[i], sortedNums[left], sortedNums[right]]);
+        // Skip duplicates of left and right pointers to avoid redundant triplets
+        while (sortedNums[left] === sortedNums[left + 1]) left++;
+        while (sortedNums[right] === sortedNums[right - 1]) right--;
+        left++;
+        right--;
+      } else if (sum < 0) {
+        left++;
+      } else if (sum > 0) {
+        right--;
+      }
+    }
+  }
+  return result;
+};
+
+// TC: O(n^2)
+// SC: O(n)