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;
}
}