LeetCode - 144. 二叉树的前序遍历 Swift & Java

给定一个二叉树，返回它的 前序 遍历。
示例:

进阶: 递归算法很简单，你可以通过迭代算法完成吗？

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/binary-tree-preorder-traversal
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

算法

swift

树结构

// Definition for a binary tree node.
public class TreeNode {
  public var val: Int
  public var left: TreeNode?
  public var right: TreeNode?
  public init(_ val: Int) {
      self.val = val
      self.left = nil
      self.right = nil
  }
}

• 递归

/**
     前序遍历：中左右
     递归
     */
    func preorderTraversal(_ root: TreeNode?) -> [Int] {
        var result = [Int]()
        helper(root, &result)
        return result
    }
    
    func helper(_ node: TreeNode?, _ result: inout [Int]) {
        guard node != nil else {
            return
        }
        result.append(node!.val)
        helper(node?.left, &result)
        helper(node?.right, &result)
    }

• 迭代

func preorderTraversal(_ root: TreeNode?) -> [Int] {
        var result = [Int]()
        if root == nil {
            return result
        }
        // 辅助栈
        var stack = [TreeNode]()
        stack.append(root!)
        while !stack.isEmpty {
            let current = stack.popLast()
            if current != nil {
                result.append(current!.val)
            } else {
                continue
            }
            // 栈：先入后出。入栈时先入右，后入左
            if current?.right != nil {
                stack.append(current!.right!)
            }
            if current?.left != nil {
                stack.append(current!.left!)
            }
        }
        return result
    }

Java

• 递归

class Solution {
    public List preorderTraversal(final TreeNode root) {
        final List result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        helper(root, result);
        return result;
    }

    public void helper(TreeNode node, final List result) {
        if (node == null) {
            return;
        }
        result.add(node.val);
        helper(node.left, result);
        helper(node.right, result);
    }
}

• 迭代

/**
     * 迭代方法
     * @param root
     * @return
     */
    public List preorderTraversal(final TreeNode root) {
        List result = new ArrayList<>();
        Stack stack = new Stack<>(); 
        if (root == null) {
            return result;
        }
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode current = stack.pop();
            result.add(current.val);
            if (current.right != null) {
                stack.push(current.right);
            }
            if (current.left != null) {
                stack.push(current.left);
            }
        }
        return result;
    }

GitHub：https://github.com/huxq-coder/LeetCode
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