BinaryTree Travel

二叉树结点定义

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

前序遍历

// Recursive
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        helper(root, res);
        return res;
    }

    private void helper(TreeNode root, List<Integer> res) {
        if (root != null) {
            res.add(root.val);
            helper(root.left, res);
            helper(root.right, res);
        }
    }
}

// Iterative
class Solution {
    public List<Integer> preorderTraversal1(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null)
            return res;
        Deque<TreeNode> stack = new ArrayDeque<>();
        stack.push(root);

        while (!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            res.add(cur.val);
            if (cur.right != null) {
                stack.push(cur.right);
            }
            if (cur.left = null) {
                stack.push(cur.left);
            }
        }
    }
}

中序遍历

// Recursive
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        helper(root, res);
        return res;
    }

    private static void helper(TreeNode root, List<Integer> res) {
        if (root != null) {
            helper(root.left, res);
            res.add(root.val);
            helper(root.right, res);
        }
    }
}

// Iterative
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null)
            return res;
        Deque<TreeNode> stack = new ArrayDeque<>();
        while (!stack.isEmpty() || root != null) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            res.add(root.val);
            root = root.right;
        }
        return res;
    }
}

后序遍历

// Recursive
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        helper(root, res);
        return res;
    }

    private static void helper(TreeNode root, List<Integer> res) {
        if (root != null) {
            helper(root.left, res);
            helper(root.right, res);
            res.add(root.val);
        }
    }
}

// Iterative
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        LinkedList<Integer> res = new LinkedList<>();
        if (root == null) {
            return res;
        }
        Deque<TreeNode> stack = new ArrayDeque<>();
        stack.add(root);

        while (!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            res.addFirst(cur.val);

            if (node.left != null) {
                stack.push(cur.left);
            }
            if (node.right != null) {
                stack.push(cur.right);
            }
        }
        return res;
    }
}