二叉树的前序遍历(非递归)

Author: mayuanucas

README.md

@@ -82,6 +82,7 @@
 | 139 | 动态规划 | [单词拆分](src/main/java/algorithm/leetcode/Solution139.java) | 中等 |
 | 141 | 链表、双指针 | [环形链表](src/main/java/algorithm/leetcode/Solution141.java) | 简单 |
 | 142 | 链表、双指针 | [环形链表 II](src/main/java/algorithm/leetcode/Solution142.java) | 中等 |
+| 144 | 栈、树 | [二叉树的前序遍历](src/main/java/algorithm/leetcode/Solution144.java) | 中等 |
 | 147 | 排序、链表 | [对链表进行插入排序](src/main/java/algorithm/leetcode/Solution147.java) | 中等 |
 | 148 | 排序、链表 | [排序链表](src/main/java/algorithm/leetcode/Solution148.java) | 中等 |
 | 152 | 数组、动态规划 | [乘积最大子序列](src/main/java/algorithm/leetcode/Solution152.java) | 中等 |

src/main/java/algorithm/leetcode/Solution144.java

@@ -1,41 +1,45 @@
 package algorithm.leetcode;
 
+import java.util.LinkedList;
+import java.util.List;
+
 /**
  * @author: mayuan
- * @desc:
- * @date: 2018/08/17
+ * @desc: 二叉树的前序遍历
+ * @date: 2019/03/09
  */
 public class Solution144 {
- public static void main(String[] args) {
-
- }
-
- public boolean hasCycle(ListNode head) {
- if (null == head) {
- return false;
+ public List<Integer> preorderTraversal(TreeNode root) {
+ List<Integer> ans = new LinkedList<>();
+ if (null == root) {
+ return ans;
  }
 
- ListNode slow = head;
- ListNode fast = head.next;
+ LinkedList<TreeNode> stack = new LinkedList<>();
+ stack.push(root);
 
- while (null != fast && null != fast.next && null != slow) {
- if (slow == fast) {
- return true;
- }
+ while (!stack.isEmpty()) {
+ TreeNode cur = stack.pop();
+ ans.add(cur.val);
 
- slow = slow.next;
- fast = fast.next.next;
+ if (null != cur.right) {
+ stack.push(cur.right);
+ }
+ if (null != cur.left) {
+ stack.push(cur.left);
+ }
  }
- return false;
+
+ return ans;
  }
 
- class ListNode {
+ public class TreeNode {
  int val;
- ListNode next;
+ TreeNode left;
+ TreeNode right;
 
- ListNode(int x) {
+ TreeNode(int x) {
  val = x;
- next = null;
  }
  }
-}
+}

src/main/java/algorithm/leetcode/Solution144a.java

@@ -0,0 +1,41 @@
+package algorithm.leetcode;
+
+/**
+ * @author: mayuan
+ * @desc:
+ * @date: 2018/08/17
+ */
+public class Solution144a {
+ public static void main(String[] args) {
+
+ }
+
+ public boolean hasCycle(ListNode head) {
+ if (null == head) {
+ return false;
+ }
+
+ ListNode slow = head;
+ ListNode fast = head.next;
+
+ while (null != fast && null != fast.next && null != slow) {
+ if (slow == fast) {
+ return true;
+ }
+
+ slow = slow.next;
+ fast = fast.next.next;
+ }
+ return false;
+ }
+
+ class ListNode {
+ int val;
+ ListNode next;
+
+ ListNode(int x) {
+ val = x;
+ next = null;
+ }
+ }
+}