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
原文：http://7371901.blog.51cto.com/7361901/1600808