68

来源：互联网发布：淘宝内部优惠券是真的吗编辑：程序博客网时间：2024/04/30 04:39

3.31

果然二叉树的后序遍历掌握的很是不好啊。

用非递归的方法不知道为啥总超时。

最后没有办法用了递归的方法。

/** * Definition of TreeNode: * public class TreeNode { *     public int val; *     public TreeNode left, right; *     public TreeNode(int val) { *         this.val = val; *         this.left = this.right = null; *     } * } */public class Solution {    /**     * @param root: The root of binary tree.     * @return: Postorder in ArrayList which contains node values.     */    public ArrayList<Integer> postorderTraversal(TreeNode root) {        ArrayList<Integer> s = new ArrayList<Integer>();       // LinkedList<TreeNode> list = new LinkedList<TreeNode>();        if(root == null){            return s;        }        s.addAll(postorderTraversal(root.left));        s.addAll(postorderTraversal(root.right));        s.add(root.val);        return s;            // write your code here    }}

再后来又研究了一下就AC了。

果然基本功要扎实啊

非递归的方式

/** * Definition of TreeNode: * public class TreeNode { *     public int val; *     public TreeNode left, right; *     public TreeNode(int val) { *         this.val = val; *         this.left = this.right = null; *     } * } */public class Solution {    /**     * @param root: The root of binary tree.     * @return: Postorder in ArrayList which contains node values.     */    public ArrayList<Integer> postorderTraversal(TreeNode root) {        ArrayList<Integer> s = new ArrayList<Integer>();        LinkedList<TreeNode> list = new LinkedList<TreeNode>();        if(root == null){            return s;        }        TreeNode bt = root;TreeNode tmp = root;while(bt != null || !list.isEmpty()){while( bt != null){//tmp = hashMap.for(;bt.left != null;bt = bt.left){list.push(bt);}while(bt != null && (bt.right ==null || bt.right == tmp)){s.add(bt.val);tmp = bt;if(list.isEmpty()){return s;}bt = list.pop();}list.push(bt);bt = bt.right;}            // write your code here        }        return s;    }}

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