力扣 144. 二叉树的前序遍历 递归 迭代
递归
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
// 递归三要素
List<Integer> result = new ArrayList<>();
preoder(root,result);
return result;
}
// 1.入参，返回值
void preoder(TreeNode root,List<Integer> result){
// 2.出栈条件
if(root == null){
return;
}
// 前序遍历，中--左--右
// 3.单层执行递归逻辑
result.add(root.val);
preoder(root.left,result);
preoder(root.right,result);
}
}
迭代
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
// 迭代
List<Integer> result = new ArrayList<>();
Stack<TreeNode> stack = new Stack<TreeNode>();
while(stack.size()>0 || root != null){
if(root!=null){
result.add(root.val);
stack.push(root);
root = root.left;
}else{
TreeNode temp = stack.pop();
root = temp.right;
}
}
return result;
}
}
