【LeetCode】二叉树先序遍历

最新推荐文章于 2022-07-20 10:11:27 发布

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#二叉树遍历

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本文详细介绍了二叉树的先序遍历算法，包括递归和非递归两种实现方式。递归方法直观易懂，直接利用递归特性进行节点访问；非递归方法则借助栈来实现，避免了递归可能导致的栈溢出问题。

递归的思想实现二叉树的先序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<Integer> list = new ArrayList<Integer>();
    public List<Integer> preorderTraversal(TreeNode root) {
        if (root == null) {
            return list;
        }

        List<Integer> list = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode curr = root;
        while (curr != null || !stack.isEmpty()) {
            if (curr != null) {
                list.add(curr.val);
                stack.push(curr);
                curr = curr.left;
            } else {
                curr = stack.pop();
                curr = curr.right;
            }
        }
        return list;
    }
}

非递归的二叉树先序遍历（借助于栈实现）

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<Integer> list = new ArrayList<Integer>();
    public List<Integer> preorderTraversal(TreeNode root) {
        if (root == null) {
            return list;
        }

        List<Integer> list = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode curr = root;
        while (curr != null || !stack.isEmpty()) {
            if (curr != null) {
                list.add(curr.val);
                stack.push(curr);
                curr = curr.left;
            } else {
                curr = stack.pop();
                curr = curr.right;
            }
        }
        return list;
    }
}

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