20190109

Author: mJackie

code/lc226.java

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+package code;
+
+import java.util.Stack;
+
+/*
+ * 226. Invert Binary Tree
+ * 题意：反转二叉树
+ * 难度：Easy
+ * 分类：Tree
+ * 思路：递归或迭代，两种思路，都很简单
+ * Tips：
+ */
+public class lc226 {
+    public class TreeNode {
+        int val;
+        TreeNode left;
+        TreeNode right;
+        TreeNode(int x) { val = x; }
+    }
+    public TreeNode invertTree(TreeNode root) {
+        //递归
+        if(root==null)
+            return null;
+        TreeNode temp = root.left;
+        root.left = root.right;
+        root.right = temp;
+        invertTree(root.left);
+        invertTree(root.right);
+        return root;
+    }
+
+    public TreeNode invertTree2(TreeNode root) {
+        //迭代
+        if(root==null)
+            return null;
+        Stack<TreeNode> st = new Stack();
+        st.add(root);
+        while(!st.isEmpty()){
+            TreeNode tn = st.pop();
+            TreeNode temp = tn.left;
+            tn.left = tn.right;
+            tn.right = temp;
+            if(tn.left!=null)
+                st.add(tn.left);
+            if(tn.right!=null)
+                st.add(tn.right);
+        }
+        return root;
+    }
+}

code/lc234.java

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+package code;
+/*
+ * 234. Palindrome Linked List
+ * 题意：判断链表是否是回文的
+ * 难度：Easy
+ * 分类：LinkedList, Two Pointers
+ * 思路：反转一般就行了，避免了空间开销
+ * Tips：很好的题，考了 Two Pointers, 还考了链表反转
+ */
+public class lc234 {
+    public class ListNode {
+        int val;
+        ListNode next;
+        ListNode(int x) { val = x; }
+    }
+
+    public boolean isPalindrome(ListNode head) {
+        if(head==null||head.next==null)
+            return true;
+        ListNode slow = head;
+        ListNode fast = head;
+        while (fast.next != null && fast.next.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+        }
+        ListNode head2 = reverse(slow.next,null);
+        slow.next = null;       //注意截断一下
+        while(head2!=null&&head!=null){
+            if(head2.val!=head.val)
+                return false;
+            head = head.next;
+            head2 = head2.next;
+        }
+        return true;
+    }
+    public ListNode reverse(ListNode head, ListNode pre){
+        ListNode next = head.next;
+        head.next = pre;
+        if(next==null)
+            return head;
+        return reverse(next, head);
+    }
+}

code/lc239.java

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+package code;
+/*
+ * 239. Sliding Window Maximum
+ * 题意：滑动窗口最大值
+ * 难度：Hard
+ * 分类：Heap
+ * 思路：用双向队列，保证队列里是递增的。单调队列，好好学习一下。
+ * Tips：与lc84做比较，84是递增栈
+ */
+import java.util.ArrayDeque;
+import java.util.Deque;
+
+public class lc239 {
+    public static void main(String[] args) {
+        int nums[] = {-95,92,-85,59,-59,-14,88,-39,2,92,94,79,78,-58,37,48,63,-91,91,74,-28,39,90,-9,-72,-88,-72,93,38,14,-83,-2,21,4,-75,-65,3,63,100,59,-48,43,35,-49,48,-36,-64,-13,-7,-29,87,34,56,-39,-5,-27,-28,10,-57,100,-43,-98,19,-59,78,-28,-91,67,41,-64,76,5,-58,-89,83,26,-7,-82,-32,-76,86,52,-6,84,20,51,-86,26,46,35,-23,30,-51,54,19,30,27,80,45,22};
+        int k = 10;
+        int[] res = maxSlidingWindow(nums,k);
+        for (int i = 0; i < res.length ; i++) {
+            System.out.print(res[i]);
+            System.out.print(',');
+        }
+    }
+    public static int[] maxSlidingWindow(int[] nums, int k) {
+        if(nums.length==0)
+            return new int[]{};
+        int[] res = new int[nums.length-k+1];
+        int cur = 0;
+        Deque<Integer> dq = new ArrayDeque();
+        for (int i = 0; i < nums.length ; i++) {
+            if( !dq.isEmpty() && dq.peekFirst()<=i-k)
+                dq.removeFirst();
+            while( !dq.isEmpty() && nums[dq.peekLast()]<=nums[i]){// removeLast 不是 First。  自己写的时候这写错了，如果是First的话，有些Case也能过
+                dq.removeLast();
+            }
+            dq.addLast(i);
+            if(i>=k-1){
+                res[cur] = nums[dq.peekFirst()];
+                cur++;
+            }
+        }
+        return res;
+    }
+}