二叉樹的中序遍歷
給定一個(gè)二叉樹，返回它的中序 遍歷。
示例：

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

輸出: [1,3,2]

非遞歸（思路更清晰）：

/**
 * 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) {
        ArrayList<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        if(root == null) return list;
        TreeNode cur = root;
        //將currentNode移動到最左邊
        while(cur != null){
            stack.push(cur);
            cur = cur.left;
        }
        while(!stack.isEmpty()){
            //根結(jié)點(diǎn)的訪問條件是只要左子樹訪問完畢，就可以訪問根結(jié)點(diǎn)（左節(jié)點(diǎn)為空或者左節(jié)點(diǎn)為上次訪問節(jié)點(diǎn)）
            cur = stack.pop();
            list.add(cur.val);
            //切換到右子樹
            cur = cur.right;
            //訪問右子樹的最左邊
            while(cur != null){
                stack.push(cur);
                cur = cur.left;
            }
        }
        
        return list;
        
    }
   
}

非遞歸：

\/**
 * 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) {
        ArrayList<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        if(root == null) return list;
        TreeNode cur = root;
        while(cur != null || !stack.isEmpty()){
            if(cur != null){
                stack.push(cur);
                cur = cur.left;
            }else{
                cur = stack.pop();
                list.add(cur.val);
                cur =cur.right;
            }
        }
        return list;
        
    }
   
}

遞歸：

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