給定一個二叉樹，返回它的先序遍歷。
示例:
輸入: [1,null,2,3]
1
\
2
/
3
輸出: [1,2,3]

樹節點類

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

非遞迴先序遍歷

     public List<Integer> preorderTraversal(TreeNode root) {
        Stack<TreeNode> s = new Stack<>();
        List<Integer> L = new ArrayList<>();
        TreeNode cur = root;
        while(cur != null || s.empty() == false){
            while(cur != null){
                L.add(cur.val);
                s.push(cur);
                cur = cur.left;
            }
            if(s.empty() == false){
                cur = s.pop();
                cur = cur.right; 
            }
        }
        return L;
    }
 

遞迴先序遍歷

    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> L = new ArrayList<>();
        return inorder(root, L);
    }

    private List<Integer> preorder(TreeNode root, List<Integer> L) {
        if (root != null) {
            L.add(root.val);
            preorder(root.left, L);
            preorder(root.right, L);
        }
        return L;
    }
 

給定一個二叉樹，返回它的中序遍歷。
示例:
輸入: [1,null,2,3]
1
\
2
/
3
輸出: [1,3,2]

非遞迴中序遍歷

    public List<Integer> inorderTraversal(TreeNode root) {
        Stack<TreeNode> s = new Stack<>();
        List<Integer> L = new ArrayList<>();
        TreeNode cur = root;
        while(cur != null || s.empty() == false){
            while(cur != null){
                s.push(cur);
                cur = cur.left;
            }
            if(s.empty() == false){
                cur = s.pop();
                L.add(cur.val);
                cur = cur.right;
            }
        }
        return L;
    }
 

遞迴中序遍歷

    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> L = new ArrayList<>();
        return inorder(root, L);
    }

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

給定一個二叉樹，返回它的後序遍歷。
示例:
輸入: [1,null,2,3]
1
\
2
/
3
輸出: [3,2,1]

非遞迴後序遍歷

    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> L = new ArrayList<>();
        Stack<TreeNode> s = new Stack<>();
        TreeNode cur = root,pre = null;;
        while(cur != null || s.empty() == false){
            while(cur != null){
                s.push(cur);
                cur = cur.left;
            }
            if(s.empty() == false){
                cur = s.peek();
                if(cur.right == null || cur.right == pre){
                    L.add(s.pop().val);
                    pre = cur;
                    cur = null;
                }else{
                    cur = cur.right;
                }
            }
        }
        return L;
    }

遞迴後序遍歷

    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> L = new ArrayList<>();
        return postorder(root, L);
    }
    private List<Integer> postorder(TreeNode root, List<Integer> L){
        if(root != null)
        {
            postorder(root.left, L);
            postorder(root.right, L);
            L.add(root.val);
        }
        return L;
    }