https://oj.leetcode.com/problems/binary-tree-preorder-traversal/

http://blog.csdn.net/linhuanmars/article/details/21428647

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        
        // Solution A:
        // return preorderTraversal_Recursion(root);
        
        // Solution B:
        return preorderTraversal_Iteration(root);
    }

    /////////////////////
    // Solution A: Recursion
    //
    private List<Integer> preorderTraversal_Recursion(TreeNode root)
    {
        List<Integer> result = new ArrayList<>();
        visit(root, result);
        return result;
    }
    
    private void visit(TreeNode node, List<Integer> result)
    {
        if (node == null)
            return;
            
        result.add(node.val);
        visit(node.left, result);
        visit(node.right, result);
    }
    
    /////////////////////
    // Solution A: Iteration
    //
    private List<Integer> preorderTraversal_Iteration(TreeNode root)
    {
        List<Integer> result = new ArrayList<>();
        
        if (root == null)
            return result;
            
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        
        while (!stack.empty())
        {
            TreeNode node = stack.pop();
            
            result.add(node.val);
            
            // Put right first
            if (node.right != null)
                stack.push(node.right);
                
            if (node.left != null)
                stack.push(node.left);
        }
        
        return result;
    }
}

[LeetCode]144 Binary Tree Preorder Traversal