二叉樹的遍歷

1、中序遍歷

（1）遞歸法

/**
 * 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> list = new ArrayList<>();
        help(root,list);
        return list;
          
    }
    private void help(TreeNode root,List<Integer> list){
        if(root != null){
            help(root.left,list);
            list.add(root.val);
            help(root.right,list);                
        }     
    }
}

（2）模擬系統棧的遞歸過程

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