leetcode 144. Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    
     2
    /
   3

return [1,2,3].

二叉树的前序遍历，非递归的写法考的比较多，方法很简单，就是把跟节点加进来，依次把右子树和左子树压入栈内，弹出的时候肯定是先弹出左子树，然后再弹出右子树，

达到先遍历跟节点，再依次遍历左子树和右子树。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        List<Integer> preorder = new ArrayList<Integer>();
        
        if (root == null) {
            return preorder;
        }
        
        stack.push(root);
        while (!stack.empty()) {
            TreeNode node = stack.pop();
            preorder.add(node.val);
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
        }
        return preorder;
    }
}

递归的方法

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

先把root压栈来，

栈非空进循环。

弹栈进数组，

右非空压栈，

左非空压栈。