java-Leetcode- Binary Tree Traversal

构造树：

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x;}
}

递归：

public class Solution {
    List<Integer> result = new ArrayList<Integer>();

    public List<Integer> inorderTraversal(TreeNode root) {
        if(root !=null){
            helper(root);
        }

        return result;
    }

    public void helper(TreeNode p){
        if(p.left!=null)
            helper(p.left);

        result.add(p.val);

        if(p.right!=null)
            helper(p.right);
    }
}

public List<Integer> inorderTraversal(TreeNode root) {
    List<Integer> result = new ArrayList<Integer>();
    if(root==null)
        return result;
    Stack<TreeNode> stack = new Stack<TreeNode>();
    stack.push(root);

    while(!stack.isEmpty()){
        TreeNode top = stack.peek();
        if(top.left!=null){
            stack.push(top.left);
            top.left=null;
        }else{
            result.add(top.val);
            stack.pop();
            if(top.right!=null){
                stack.push(top.right);
            }
        }
    }

    return result;
}