题目描述
https://leetcode-cn.com/problems/binary-tree-preorder-traversal/
解法
迭代法
有了前面的中序遍历的思考经验,我们可以很快地写出如下代码:
class Solution {
List<Integer> list = new ArrayList<>();
public List<Integer> preorderTraversal(TreeNode root) {
preorderTraversal(root,list);
return list;
}
public void preorderTraversal(TreeNode root, List<Integer>list){
//迭代法,利用栈
if(root==null) return;
Stack<TreeNode> stack = new Stack<>();//创建一个栈
TreeNode cur = root;
boolean flag = false;//表示当前的结点还没有完成任务
while(cur!=null|| !stack.isEmpty()){
while(cur!=null){
stack.push(cur);
//访问当前的值
list.add(cur.val);
cur = cur.left;
}
cur = stack.pop();
cur = cur.right;
}
}
}
递归写法
当然我们还是递归写法简洁明了:
class Solution {
//中序递归:
List<Integer> list = new ArrayList<>();
public List<Integer> preorderTraversal(TreeNode root) {
preorderTraversal(root,list);
return list;
}
public void preorderTraversal(TreeNode root, List<Integer> list){
if(root==null) return;
//前序遍历
list.add(root.val);
preorderTraversal(root.left,list);
preorderTraversal(root.right,list);
}
}