For example:
Given binary tree {1,#,2,3},
1
\
2
/
3
return [3,2,1].
输出一颗树的后序遍历序列。我们可以用递归和迭代两种方法。用迭代时我们借助堆栈,因为是后序遍历,顺序为左-右-根,因此我们只需要通过根-右-左的顺序进行遍历,然后将序列翻转就可以了。代码如下:
递归:
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public List<Integer> postorderTraversal(TreeNode root) { List<Integer> list = new ArrayList<Integer>(); if(root == null) return list; getPostorder(root, list); return list; } public void getPostorder(TreeNode root, List<Integer> list) { if(root == null) return; getPostorder(root.left, list); getPostorder(root.right, list); list.add(root.val); } }
迭代:
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public List<Integer> postorderTraversal(TreeNode root) { LinkedList<Integer> list = new LinkedList<Integer>(); Stack<TreeNode> stack = new Stack<TreeNode>(); if(root == null) return list; stack.add(root); while(!stack.isEmpty()) { TreeNode node = stack.pop(); list.addFirst(node.val); if(node.left != null) { stack.push(node.left); } if(node.right != null) { stack.push(node.right); } } return list; } }