Solved linkedList, array n string leetcode problems.

Author: jyotisirswa

DataStrucutureQuestions/Array/16. 3Sum Closest.playground/Contents.swift

@@ -0,0 +1,37 @@
+import UIKit
+
+//Two pointer approach https://leetcode.com/problems/3sum-closest/description/
+/* In the solution above, sorting the array takes O(nlog(n))  and the nested loop takes O(n2)  to find the triplet. Here, n
+ is the number of elements in the input array. Therefore, the total time complexity of this solution is O(nlog(n)+n2 ) , which simplifies to O(n
+ 2). */
+
+func threeSumClosest(_ nums: [Int], _ target: Int) -> Int {
+     let sortedArray = nums.sorted()
+     let length = sortedArray.count
+
+     var difference : Int = .max
+     var result = 0
+
+     for index in 0..<length - 1 {
+         var low = index + 1
+         var high = length - 1
+
+         while low < high {
+             let sum = sortedArray[index] + sortedArray[low] + sortedArray[high]
+             sum > target ? (high -= 1) : (low += 1)
+
+             let closeDifference =  abs(sum - target)
+
+             if closeDifference < difference {
+                 difference =  closeDifference
+                 result = sum
+             }
+
+         }
+     }
+     return result
+}
+let nums = [-1,2,1,-4]
+print(threeSumClosest(nums, 1))
+
+

DataStrucutureQuestions/Array/16. 3Sum Closest.playground/contents.xcplayground

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='ios' buildActiveScheme='true' importAppTypes='true'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>

DataStrucutureQuestions/LinkedList/19. Remove Nth Node From End of List.playground/Contents.swift

@@ -1,5 +1,9 @@
 import UIKit
 
+//Problem:-  https://leetcode.com/problems/remove-nth-node-from-end-of-list/description/
+
+//Leetcode Solution : https://leetcode.com/problems/remove-nth-node-from-end-of-list/solutions/4180205/19-remove-nth-node-from-end-of-list-time-complexity-is-o-n-space-complexity-o-1/
+
 
 public class ListNode {
     public var val : Int?
@@ -27,25 +31,26 @@ extension ListNode : CustomStringConvertible {
 }
 
 func removeNthFromEnd(_ head: ListNode?, _ n: Int) -> ListNode? {
-    var dummyNode = ListNode(0, head)
-    var left = dummyNode
+    var left = head
     var right = head
     var index = n
     // advance right to nth position
     while index > 0 {
         right = right?.next
         index -= 1
     }
+    if right == nil {
+        return head?.next
+    }
     /* then advance both right and left now they are nth postions apart
-    # when right gets to nil, left will be just before the item to be deleted */
-    while right != nil {
+     # when right gets to nil, left will be just before the item to be deleted */
+    while right?.next != nil {
         right = right?.next
-        left = left.next!
+        left = left?.next
     }
     // delete the node
-    left.next = left.next?.next
-
-    return dummyNode.next
+    left?.next = left?.next?.next
+    return head
 }
 
 let ll = ListNode(1, ListNode(1, ListNode(2, ListNode(3, ListNode(6, ListNode(4, ListNode(5, nil)))))))

DataStrucutureQuestions/String/125. Valid Palindrome.playground/Contents.swift

@@ -0,0 +1,35 @@
+import UIKit
+
+//MARK: -  https://leetcode.com/problems/valid-palindrome/?envType=study-plan-v2&envId=top-interview-150
+/*Time complexity:
+The time complexity is O(n), where n is the number of characters in the string. However, our algorithm will only run O(n/2)times, since two pointers are traversing toward each other.
+
+Space complexity:
+The space complexity is O(1), since we use constant space to store two indexes. */
+
+func isPalindrome(_ s: String) -> Bool {
+    var lower = s.startIndex
+    var upper = s.index(before: s.endIndex)
+    while lower < upper {
+        if !s[lower].isLetter, !s[lower].isNumber {
+            lower = s.index(after: lower)
+            continue
+        }
+        
+        if !s[upper].isLetter, !s[upper].isNumber {
+            upper = s.index(before: upper)
+            continue
+        }
+        
+        if s[lower].lowercased() != s[upper].lowercased() {
+            return false
+        }
+        lower = s.index(after: lower)
+        upper = s.index(before: upper)
+    }
+    return true
+}
+
+let string = "A man, a plan, a canal: Panama"
+print(isPalindrome(string))
+

DataStrucutureQuestions/String/125. Valid Palindrome.playground/contents.xcplayground

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='ios' buildActiveScheme='true' importAppTypes='true'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>