Adding all the solutions

Intersection-of-linked-lists.java

@@ -0,0 +1,63 @@
+// Solution -1
+
+// TimeComplexity: O(m+n)
+// SpaceComplexity: O(1)
+public class Solution {
+    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
+        ListNode first = headA;
+        ListNode second = headB;
+        int m =0;
+        int n =0;
+        while(first !=null) {
+            m++;
+            first = first.next;
+        }
+        while(second !=null) {
+            n++;
+            second = second.next;
+        }
+        if(m>n) {
+            for(int i=0; i<m-n; i++) {
+                headA=headA.next;
+            }
+        } else {
+            for(int i=0; i<n-m; i++) {
+                headB=headB.next;
+            }
+        }
+
+        while(headA!=null) {
+            if(headA==headB) return headA;
+            headA=headA.next;
+            headB=headB.next;
+        }
+        return null;
+    }
+}
+
+
+// Solution -2 
+
+
+// TimeComplexity: O(m+n)
+// SpaceComplexity: O(m+n)
+
+public class Solution {
+    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
+        HashSet<ListNode> set = new HashSet<>();
+        while(headA!=null) {
+            set.add(headA);
+            headA= headA.next;
+        }
+
+        while(headB!=null) {
+            if(set.contains(headB)) {
+                return headB;
+            }
+            set.add(headB);
+            headB= headB.next;
+        }
+        return null;
+
+    }
+}

binary-search-tree-operator .java

@@ -0,0 +1,27 @@
+class BSTIterator {
+    Stack<TreeNode> st;
+    public BSTIterator(TreeNode root) {
+        st = new Stack<>();
+        dfs(root);  
+    }
+
+    private void dfs(TreeNode root) {
+        while(root!=null) {
+            st.add(root);
+            root = root.left;
+        }
+    } 
+
+    // TimeComplexity: O(h) but amortized O(1) 
+    public int next() {
+        TreeNode curr = st.pop();
+        dfs(curr.right);
+        return curr.val;
+
+    }
+
+    // TimeComplexity: O(1) 
+    public boolean hasNext() {  
+        return !st.isEmpty(); 
+    }
+}

delete-without-head-pointer.java

@@ -0,0 +1,35 @@
+// Solution - 1 
+
+// Time Complexity: O(1)
+// Space Complexity: O(1)
+
+class Solution {
+    public void deleteNode(Node del_node) {
+        // code here
+        Node curr = del_node;
+        Node temp = curr.next;
+        curr.data = curr.next.data;
+        curr.next = curr.next.next;
+        temp.next = null;
+    }
+}
+
+// Solution - 2
+
+// Time Complexity: O(n)
+// Space Complexity: O(1)
+
+class Solution {
+    public void deleteNode(Node del_node) {
+        // code here
+        Node curr = del_node;
+        Node temp = curr.next;
+        while(temp.next!=null) {
+            curr.data = temp.data;
+            curr = temp;
+            temp = temp.next;
+        }
+        curr.data = temp.data;
+        curr.next = null;
+    }
+}

reorder-list.java

@@ -0,0 +1,72 @@
+// Solution -1
+
+// TimeComplexity: O(n)
+// SpaceComplexity: O(1)
+class Solution {
+    public void reorderList(ListNode head) {
+        // 1. Find the middle
+        // 2. Reverse the second half
+        // 3. Merge both of them
+        ListNode slow = head;
+        ListNode fast = head;
+
+    while(fast.next!=null && fast.next.next!=null) {
+        slow = slow.next;
+        fast = fast.next.next;
+    }
+
+    ListNode curr = slow.next;
+    ListNode prev = null;
+    while(curr !=null) {
+        ListNode temp = curr.next;
+        curr.next = prev;
+        prev = curr;
+        curr = temp;
+    }
+    slow.next = null;
+    fast = prev;
+    slow = head;
+
+    while(fast!=null){
+        ListNode temp = slow.next;
+        slow.next = fast;
+        fast = fast.next;
+        slow.next.next = temp;
+        slow = temp;
+    }
+    }
+}
+
+
+
+// Solution -2
+
+// TimeComplexity: O(n^2)
+// SpaceComplexity: O(1)
+
+class Solution {
+    public void reorderList(ListNode head) {
+        int n=0;
+        ListNode nxt = head;
+        while(nxt!=null) {
+            n++;
+            nxt = nxt.next;
+        }
+        nxt = head;
+
+        while(nxt.next!=null && nxt.next.next !=null) {
+            ListNode curr = nxt;
+            ListNode fast = nxt;
+            nxt = nxt.next;
+            n = n-2;
+            int i =0;
+            while(i<n) {
+                fast = fast.next;
+                i++;
+            }
+            curr.next = fast.next;
+            fast.next = null;
+            curr.next.next = nxt;
+        }   
+    }
+}