#145 后序遍历

Author: Isaaclot

main/src/tree/LC_145_BinaryTreePostOrderTraversal.java

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+package tree;
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Stack;
+
+/**
+ * 后序遍历：左右根
+ *
+ * @author liangqian0723@gmail.com
+ * @since 2020/3/6 12:42 AM
+ */
+public class LC_145_BinaryTreePostOrderTraversal {
+    public List<Integer> postOrderTraversal_recur(TreeNode root) {
+        List<Integer> result = new ArrayList<>();
+        if (root == null) {
+            return result;
+        }
+        if (root.left != null) {
+            result.addAll(postOrderTraversal_recur(root.left));
+        }
+        if (root.right != null) {
+            result.addAll(postOrderTraversal_recur(root.right));
+        }
+        result.add(root.val);
+        return result;
+    }
+
+    /**
+     * 逆序输出到链表结构中: 比如一个根节点、左节点、右节点；先将根节点放在链表头部，再将右节点放在链表头部，最后将左节点放在链表头部；最终顺序就是 左 右 根 (后序遍历)
+     */
+    public List<Integer> postOrderTraversal_loop(TreeNode head) {
+        LinkedList<Integer> result = new LinkedList<>();
+        if (null == head) return result;
+        Stack<TreeNode> stack = new Stack<>();
+        stack.add(head);
+        TreeNode lastNode = head;
+        while (!stack.isEmpty()) {
+            TreeNode cur = stack.pop();
+            result.addFirst(cur.val); //
+            if (cur.left != null) {
+                stack.add(cur.left);
+            }
+            if (cur.right != null) {
+                stack.add(cur.right);
+            }
+        }
+        return result;
+    }
+}

main/test/tree/LC_145_BinaryTreePostOrderTraversalTest.java

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+package tree;
+
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.List;
+
+import static org.hamcrest.core.Is.is;
+import static org.junit.Assert.assertThat;
+
+/**
+ * @author liangqian0723@gmail.com
+ * @since 2020/3/6 12:38 AM
+ */
+public class LC_145_BinaryTreePostOrderTraversalTest {
+    @Test
+    public void test_postorderTraversal_recur() {
+        TreeNode root = new TreeNode(1);
+        TreeNode node2 = new TreeNode(2);
+        TreeNode node3 = new TreeNode(3);
+        root.right = node2;
+        node2.left = node3;
+        LC_145_BinaryTreePostOrderTraversal binaryTreePostOrderTraversal = new LC_145_BinaryTreePostOrderTraversal();
+        List<Integer> result = binaryTreePostOrderTraversal.postOrderTraversal_recur(root);
+        List<Integer> expected = Arrays.asList(3, 2, 1);
+        assertThat(result, is(expected));
+    }
+
+    @Test
+    public void test_postorderTraversal_loop() {
+        TreeNode root = new TreeNode(1);
+        TreeNode node2 = new TreeNode(2);
+        TreeNode node3 = new TreeNode(3);
+        root.right = node2;
+        node2.left = node3;
+        LC_145_BinaryTreePostOrderTraversal binaryTreePostOrderTraversal = new LC_145_BinaryTreePostOrderTraversal();
+        List<Integer> result = binaryTreePostOrderTraversal.postOrderTraversal_loop(root);
+        List<Integer> expected = Arrays.asList(3, 2, 1);
+        assertThat(result, is(expected));
+    }
+}