LeetCode: Binary Tree Preorder Traversal 解题报告

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].

Note: Recursive solution is trivial, could you do it iteratively?

SOLUTION1&2:

递归及非递归解法：

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    // sol1:
    public List<Integer> preorderTraversal1(TreeNode root) {
        List<Integer> ret = new ArrayList<Integer>();
        
        rec(root, ret);
        return ret;
    }
    
    public void rec(TreeNode root, List<Integer> ret) {
        if (root == null) {
            return;
        }
        
        ret.add(root.val);
        rec(root.left, ret);
        rec(root.right, ret);
    }
    
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ret = new ArrayList<Integer>();
        
        if (root == null) {
            return ret;
        }        
        
        Stack<TreeNode> s = new Stack<TreeNode>();
        s.push(root);
        
        while (!s.isEmpty()) {
            TreeNode cur = s.pop();
            ret.add(cur.val);
            
            if (cur.right != null) {
                s.push(cur.right);
            }
            
            if (cur.left != null) {
                s.push(cur.left);
            }
        }
        
        return ret;
    }
}

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/tree/PreorderTraversal.java