二叉树小结

二叉树

树的遍历(如何遍历，如何利用特性问题)

前序遍历（中前后）

递归

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

    void inorder(TreeNode root, List list) {
        if (root == null) {
            return;
        }
        inorder(root.left, list);
        list.add(root.val);             // 注意这一句
        inorder(root.right, list);
    }
}

迭代

class Solution {
    public List preorderTraversal(TreeNode root) {
        List res = new ArrayList<>();
        if (root == null) return res;
        Stack stack = new Stack<>();
        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);
        }
        return res;
    }

}

中序遍历（前中后）

递归

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

    void inorder(TreeNode root, List list) {
        if (root == null) {
            return;
        }
        inorder(root.left, list);
        list.add(root.val);             // 注意这一句
        inorder(root.right, list);
    }
}

迭代

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

}

后序遍历（前后中）

递归

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

    void postorder(TreeNode root, List list) {
        if (root == null) {
            return;
        }
        postorder(root.left, list);
        postorder(root.right, list);
        list.add(root.val);             // 注意这一句
    }
}

迭代

class Solution {
    public List postorderTraversal(TreeNode root) {
        List res = new ArrayList<>();
        if (root == null) return res;
        Stack stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode cur = stack.pop();
            res.add(cur.val);
            if(cur.left != null) stack.push(cur.left);
            if(cur.right != null) stack.push(cur.right);
        }
        Collections.reverse(res);
        return res;
    }
}

利用遍历的特性

• 涉及到二叉树的构造，无论普通二叉树还是二叉搜索树一定前序，都是先构造中节点。
• 求普通二叉树的属性，一般是后序，一般要通过递归函数的返回值做计算。
• 求二叉搜索树的属性，一定是中序了，要不白瞎了有序性了。