ValidTriangleNumber

Signed-off-by: Gopal S Akshintala <[email protected]>

Author: overfullstack

ds-algo/src/main/java/practice/ThreeSum.java

@@ -9,21 +9,22 @@ public class ThreeSum {
 	public List<List<Integer>> threeSum(int[] nums) {
 		Arrays.sort(nums);
 		var result = new ArrayList<List<Integer>>();
-		for (var i = 0; i < nums.length; i++) {
-			if (i == 0 || nums[i] != nums[i - 1]) {
-				var j = i + 1;
-				var k = nums.length - 1;
-				while (j < k) {
-					var sum = nums[i] + nums[j] + nums[k];
+		for (var left1 = 0; left1 < nums.length; left1++) {
+			if (left1 == 0 || nums[left1] != nums[left1 - 1]) {
+        // ! init left2, right in the inner loop
+				var left2 = left1 + 1;
+				var right = nums.length - 1; 
+				while (left2 < right) {
+					var sum = nums[left1] + nums[left2] + nums[right];
 					if (sum > 0) {
-						k--;
+						right--;
 					} else if (sum < 0) {
-						j++;
+						left2++;
 					} else {
-						result.add(Arrays.asList(nums[i], nums[j], nums[k]));
-						do {
-							j++;
-						} while (nums[j] == nums[j - 1] && j < k);
+						result.add(Arrays.asList(nums[left1], nums[left2], nums[right]));
+            do {
+              left2++;
+            } while (nums[left2] == nums[left2 - 1] && left2 < right);
 					}
 				}
 			}

ds-algo/src/main/java/practice/ValidTriangleNumber.java

@@ -0,0 +1,38 @@
+package practice;
+
+/* 26 Oct 2025 12:14 */
+
+import java.util.Arrays;
+
+import static java.lang.IO.println;
+
+/**
+ * [611. Valid Triangle Number](https://leetcode.com/problems/valid-triangle-number/)
+ */
+public class ValidTriangleNumber {
+  public int triangleNumber(int[] nums) {
+    Arrays.sort(nums);
+    var count = 0;
+    // ! Fix the largest side (right) and find pairs of smaller sides
+    for (var right = nums.length - 1; right >= 2; right--) {
+      var left1 = 0;
+      var left2 = right - 1;
+      while (left1 < left2) {
+        if (nums[left1] + nums[left2] > nums[right]) {
+          // ! All elements between left1 and left2 form valid triangles with left2 and right
+          count += left2 - left1;
+          left2--;
+        } else {
+          left1++;
+        }
+      }
+    }
+    return count;
+  }
+
+  static void main() {
+    var obj = new ValidTriangleNumber();
+    println(obj.triangleNumber(new int[] {2, 2, 3, 4})); // 3
+    println(obj.triangleNumber(new int[] {4, 2, 3, 4})); // 4
+  }
+}