Solve 7th week problems

Author: bky373

binary-tree-level-order-traversal/bky373.java

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+/**
+ * time: O(N)
+ * space: O(N)
+ */
+class Solution {
+
+    public List<List<Integer>> levelOrder(TreeNode root) {
+        List<List<Integer>> result = new ArrayList<List<Integer>>();
+        if (root == null) {
+            return result;
+        }
+
+        int level = 0;
+        Queue<TreeNode> que = new LinkedList<TreeNode>();
+        que.add(root);
+
+        while (!que.isEmpty()) {
+            result.add(new ArrayList<Integer>());
+
+            int len = que.size();
+            for (int i = 0; i < len; i++) {
+                TreeNode cur = que.poll();
+                result.get(level)
+                      .add(cur.val);
+
+                if (cur.left != null) {
+                    que.add(cur.left);
+                }
+                if (cur.right != null) {
+                    que.add(cur.right);
+                }
+            }
+            level++;
+        }
+        return result;
+    }
+}

lowest-common-ancestor-of-a-binary-search-tree/bky373.java

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+/**
+ * time: O(N)
+ * space: O(N)
+ */
+class Solution {
+
+    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
+        int curVal = root.val;
+        int pVal = p.val;
+        int qVal = q.val;
+
+        if (pVal > curVal && qVal > curVal) {
+            return lowestCommonAncestor(root.right, p, q);
+        } else if (pVal < curVal && qVal < curVal) {
+            return lowestCommonAncestor(root.left, p, q);
+        } else {
+            return root;
+        }
+    }
+}

remove-nth-node-from-end-of-list/bky373.java

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+/**
+ * time: O(N)
+ * space: O(1)
+ */
+class Solution {
+
+    public ListNode removeNthFromEnd(ListNode head, int n) {
+        int sz = 0;
+        ListNode current = head;
+
+        while (current != null) {
+            current = current.next;
+            sz++;
+        }
+
+        if (sz == n) {
+            return head.next;
+        }
+
+        int removedIndex = sz - n;
+        current = head;
+        for (int i = 0; i < removedIndex-1; i++) {
+            current = current.next;
+        }
+        current.next = current.next.next;
+        return head;
+    }
+}

reorder-list/bky373.java

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+/**
+ * time: O(N)
+ * space: O(N)
+ */
+class Solution {
+
+    public void reorderList(ListNode head) {
+        if (head.next == null) {
+            return;
+        }
+        ListNode reorderHead = head;
+        List<ListNode> nodes = new ArrayList<>();
+        while (head != null) {
+            ListNode next = head.next;
+            head.next = null;
+            nodes.add(head);
+            head = next;
+        }
+
+        int left = 0;
+        int right = nodes.size() - 1;
+        while (left < right) {
+            reorderHead.next = nodes.get(right);
+            right--;
+            left++;
+            if (left <= right && left < nodes.size()) {
+                reorderHead = reorderHead.next;
+                reorderHead.next = nodes.get(left);
+                reorderHead = reorderHead.next;
+            }
+        }
+    }
+}

validate-binary-search-tree/bky373.java

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+/**
+ * time: O(N)
+ * space: O(N)
+ */
+class Solution {
+
+    public boolean isValidBST(TreeNode root) {
+        return dfs(root, null, null);
+    }
+
+    public boolean dfs(TreeNode cur, Integer low, Integer high) {
+        if (cur == null) {
+            return true;
+        }
+        if (low != null && cur.val <= low) {
+            return false;
+        }
+        if (high != null && cur.val >= high) {
+            return false;
+        }
+        return dfs(cur.left, low, cur.val) && dfs(cur.right, cur.val, high);
+    }
+}