给定一个二叉树,返回它的先序遍历。
示例:
输入: [1,null,2,3]
1
\
2
/
3
输出: [1,2,3]
树节点类
public class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int x) { val = x; }
}
非递归先序遍历
public List<Integer> preorderTraversal(TreeNode root) {
Stack<TreeNode> s = new Stack<>();
List<Integer> L = new ArrayList<>();
TreeNode cur = root;
while(cur != null || s.empty() == false){
while(cur != null){
L.add(cur.val);
s.push(cur);
cur = cur.left;
}
if(s.empty() == false){
cur = s.pop();
cur = cur.right;
}
}
return L;
}
递归先序遍历
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> L = new ArrayList<>();
return inorder(root, L);
}
private List<Integer> preorder(TreeNode root, List<Integer> L) {
if (root != null) {
L.add(root.val);
preorder(root.left, L);
preorder(root.right, L);
}
return L;
}
给定一个二叉树,返回它的中序遍历。
示例:
输入: [1,null,2,3]
1
\
2
/
3
输出: [1,3,2]
非递归中序遍历
public List<Integer> inorderTraversal(TreeNode root) {
Stack<TreeNode> s = new Stack<>();
List<Integer> L = new ArrayList<>();
TreeNode cur = root;
while(cur != null || s.empty() == false){
while(cur != null){
s.push(cur);
cur = cur.left;
}
if(s.empty() == false){
cur = s.pop();
L.add(cur.val);
cur = cur.right;
}
}
return L;
}
递归中序遍历
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> L = new ArrayList<>();
return inorder(root, L);
}
private List<Integer> inorder(TreeNode root, List<Integer> L) {
if (root != null) {
inorder(root.left, L);
L.add(root.val);
inorder(root.right, L);
}
return L;
}
给定一个二叉树,返回它的后序遍历。
示例:
输入: [1,null,2,3]
1
\
2
/
3
输出: [3,2,1]
非递归后序遍历
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> L = new ArrayList<>();
Stack<TreeNode> s = new Stack<>();
TreeNode cur = root,pre = null;;
while(cur != null || s.empty() == false){
while(cur != null){
s.push(cur);
cur = cur.left;
}
if(s.empty() == false){
cur = s.peek();
if(cur.right == null || cur.right == pre){
L.add(s.pop().val);
pre = cur;
cur = null;
}else{
cur = cur.right;
}
}
}
return L;
}
递归后序遍历
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> L = new ArrayList<>();
return postorder(root, L);
}
private List<Integer> postorder(TreeNode root, List<Integer> L){
if(root != null)
{
postorder(root.left, L);
postorder(root.right, L);
L.add(root.val);
}
return L;
}