二叉树的中序遍历

简书来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/binary-tree-inorder-traversal

给定一个二叉树，返回它的中序 遍历。

示例:

输入: [1,null,2,3]
   1
    \
     2
    /
   3

输出: [1,3,2]

递归解法：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        inorder(ans,root);
        return ans;
    }

     private void inorder(List<Integer> ans,TreeNode root){
        if (root == null)
            return;
        inorder(ans,root.left);
        ans.add(root.val);
        inorder(ans,root.right);
    }
}

迭代解法：

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


    private void addLeft(Stack<TreeNode> stack,TreeNode root){
        while(root != null){
            stack.push(root);
            root = root.left;
        }
    }
}