LeetCode之二叉树的前序遍历

**题目：**给定一个二叉树，返回它的前序遍历
示例：

方法一：递归

class Solution {
	public List<Integer> preorderTraversal(TreeNode root) {
			List<Integer> list=new ArrayList<Integer>();
			if(root==null)return list;
			preOrder(root,list);
			return list;
	}
	public void preOrder(TreeNode root,List<Integer> list){
			if(root==null)return;
			list.add(root.val);
			preOrder(root.left,list);
			preOrder(root.right,list);
			
	}
}

方法二：迭代

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