94. Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes' values.

Example:

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

Output: [1,3,2]


用Stack，左根右，push左，左=左。根。

/**
 * 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> result = new ArrayList<>();
        Stack<TreeNode> s = new Stack<>();
        TreeNode p = root;

        while (!s.empty() || p != null) {
            if (p != null) {
                s.push(p);
                p = p.left;
            } else {
                p = s.pop();
                result.add(p.val);
                p = p.right;
            }
        }
        return result;
    }
}