BinaryTreePreorderTraversal
BinaryTreePreorderTraversal.java 源码
package datastructure.tree.leetcode;
import java.util.ArrayList;
import java.util.List;
import java.util.Stack;
/**
* @author roseduan
* @time 2020/9/15 9:02 下午
* @description 二叉树前序遍历
*/
public class BinaryTreePreorderTraversal {
private List<Integer> res = new ArrayList<>();
/**
* 递归
*/
public List<Integer> preorderTraversal(TreeNode root) {
if (root == null) {
return res;
}
res.add(root.val);
preorderTraversal(root.left);
preorderTraversal(root.right);
return res;
}
/**
* 迭代
*/
public List<Integer> preorderTraversal2(TreeNode root) {
List<Integer> list = new ArrayList<>();
if (root == null) {
return list;
}
Stack<TreeNode> stack = new Stack<>();
TreeNode cur = root;
stack.push(cur);
while (!stack.empty()) {
cur = stack.pop();
list.add(cur.val);
if (cur.right != null) {
stack.push(cur.right);
}
if (cur.left != null) {
stack.push(cur.left);
}
}
return list;
}
private static class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int val, TreeNode left, TreeNode right) {
this.val = val;
this.left = left;
this.right = right;
}
}
}
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