20190129

code/lc103.java

+package code;
+/*
+ * 103. Binary Tree Zigzag Level Order Traversal
+ * 题意：蛇形层次遍历
+ * 难度：Medium
+ * 分类：Stack, Tree, Breadth-first Search
+ * 思路：层次遍历，加了个flag每次是在list头添加或尾添加
+ * Tips：Bingo!
+ */
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Queue;
+
+public class lc103 {
+
+    public class TreeNode {
+        int val;
+        TreeNode left;
+        TreeNode right;
+        TreeNode(int x) { val = x; }
+    }
+
+    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
+        List<List<Integer>> res = new ArrayList<>();
+        if(root==null) return res;
+        Queue<TreeNode> q = new ArrayDeque();
+        q.add(root);
+        boolean flag = true;
+        while(!q.isEmpty()){
+            int size = q.size();
+            List<Integer> ls = new ArrayList<>();
+            while(size>0){
+                TreeNode temp = q.remove();
+                if(flag) ls.add(temp.val);
+                else ls.add(0,temp.val);    //在头部添加
+                if(temp.left!=null) q.add(temp.left);
+                if(temp.right!=null) q.add(temp.right);
+                size--;
+            }
+            res.add(ls);
+            flag = !flag;
+        }
+        return res;
+    }
+}

code/lc116.java

+package code;
+/*
+ * 116. Populating Next Right Pointers in Each Node
+ * 题意：设置二叉树的next指针，指向同层右侧节点
+ * 难度：Medium
+ * 分类：Tree, Depth-first Search
+ * 思路：自己写的递归，中间有冗余。迭代方法和较优的递归会利用先前已经设置的next指针，做下一步操作
+ * Tips：复习时多看一下，自己没想起来利用已经设置好的指针
+ */
+public class lc116 {
+    public class TreeLinkNode {
+        int val;
+        TreeLinkNode left, right, next;
+        TreeLinkNode(int x) { val = x; }
+    }
+    public void connect(TreeLinkNode root) {
+        if(root==null) return;
+        helper(root.left, root.right);
+    }
+
+    public void helper(TreeLinkNode root1, TreeLinkNode root2){//较慢，也能过
+        if( root1==null || root2==null ) return;
+        root1.next = root2;
+        helper(root1.left, root1.right);
+        helper(root1.right, root2.left);    //会重复设置
+        helper(root2.left,root2.right);
+    }
+
+    public void connect2(TreeLinkNode root) {
+        while(root!=null){
+            TreeLinkNode start = root;
+            while(start!=null){
+                if(start.left!=null){
+                    start.left.next = start.right;  //设置下一层的next指针
+                    if(start.next!=null){
+                        start.right.next = start.next.left;
+                    }
+                }
+                start = start.next;
+            }
+            root = root.left;
+        }
+    }
+}

code/lc118.java

+package code;
+
+import java.util.List;
+
+public class lc118 {
+    public List<List<Integer>> generate(int numRows) {
+        while(numRows>0){
+            
+        }
+    }
+}

readme.md

@@ -69,6 +69,7 @@ Language: Java
 | 084 [Java](./code/lc84.java)
 | 085 [Java](./code/lc85.java)
 | 088 [Java](./code/lc88.java)
+| 091 [Java](./code/lc91.java)
 | 094 [Java](./code/lc94.java)
 | 096 [Java](./code/lc96.java)
 | 098 [Java](./code/lc98.java)
@@ -76,6 +77,7 @@ Language: Java
 | 102 [Java](./code/lc102.java)
 | 104 [Java](./code/lc104.java)
 | 105 [Java](./code/lc105.java)
+| 108 [Java](./code/lc108.java)
 | 114 [Java](./code/lc114.java)
 | 121 [Java](./code/lc121.java)
 | 122 [Java](./code/lc122.java)