package 树的遍历.q144_二叉树的前序遍历;
public class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int x) {
val = x;
}
}
package 树的遍历.q144_二叉树的前序遍历;
import java.util.ArrayList;
import java.util.List;
import java.util.Stack;
/**
* 非递归法 o(n)
*/
public class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> rs = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
while (!stack.empty() || root != null) {
while (root != null) {
rs.add(root.val);
stack.push(root);
root = root.left;
}
root = stack.pop();
root = root.right;
}
return rs;
}
public static void main(String[] args) {
TreeNode root = new TreeNode(1);
TreeNode t1 = new TreeNode(2);
root.right = t1;
TreeNode t2 = new TreeNode(3);
t1.left = t2;
new Solution().preorderTraversal(root);
}
}