Travesal

Preorder 前序遍历 - 递归

public List<Integer> preorder(TreeNode root) {
    List<Integer> result = new ArrayList<Integer>();
    preOrderTravesalHelper(result, root);
    return result;
}

private void preOrderTravesalHelper(List<Integer> re, TreeNode root) {
    if (root == null) {
        return;
    }
    re.add(root.val);
    preOrderTravesalHelper(re, root.left);
    preOrderTravesalHelper(re, root.right);
}

Preorder 前序遍历 - 非递归

public List<Integer> preorderNonRecursive(TreeNode root) {
    List<Integer> result = new ArrayList<Integer>();
    Stack<TreeNode> stack = new Stack<TreeNode>();

    if (root != null) {
        stack.push(root);
        while (!stack.isEmpty) {
            root = stack.pop();
            result.add(root.val);
            if (root.right != null) {
                stack.push(root.right);
            }
            if (root.left != null) {
                stack.push(root.left);
            }
        }
    }
    return reuslt;
}

Inorder 中序遍历 - 递归

public List<Integer> inorder(TreeNode root) {
    List<Integer> result = new ArrayList<Integer>();
    if (root == null) {
        return result;
    }
    inOrderTravesalHelper(result, root);
    return result;
}

private void inOrderTravesalHelper(List<Integer> re, TreeNode node) {
    if (node == null) {
        return;
    }
    inOrderTravesalHelper(re, node.left);
    re.add(node.val);
    inOrderTravesalHelper(re, node.right);
}

Inorder 中序遍历 - 非递归

public List<Integer> inorderNonRecursive(TreeNode root) {
    List<Integer> result = new ArrayList<Integer>();
    Stack<TreeNode> stack = new Stack<TreeNode>();

    while (root != null || !stack.isEmpty()) {
        if (root != null) {
            stack.push(root);
            root = root.left;
        } else {
            root = stack.pop();
            result.add(root.val);
            root = root.right;
        }
    }
    return result;
}

广度优先BFS - 非递归

解法： 用队列依次保存左右节点

public void breadthFirstTraversal(TreeNode root) {
    ArrayDeque<TreeNode> queue = new ArrayDeque<TreeNode>();
    TreeNode node = null;
    queue.add(root);
    while (!queue.isEmpty()) {
        node = queue.remove();
        visit(node);
        if (node.left != null) {
            queue.add(node.left);
        }
        if (node.right != null) {
            queue.add(node.right);
        }
    }
}