Invert a binary tree.html
4 / \ 2 7 / \ / \ 1 3 6 9
tonode
4 / \ 7 2 / \ / \ 9 6 3 1
Trivia:
This problem was inspired by this original tweet by Max Howell:post
Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.
這道題讓咱們翻轉二叉樹,是樹的基本操做之一,不算難題。最下面那句話實在有些木有節操啊,不知道是Google說給誰的。反正這道題確實難度不大,能夠用遞歸和非遞歸兩種方法來解。先來看遞歸的方法,寫法很是簡潔,五行代碼搞定,交換當前左右節點,並直接調用遞歸便可,代碼以下:this
// Recursion class Solution { public: TreeNode* invertTree(TreeNode* root) { if (!root) return NULL; TreeNode *tmp = root->left; root->left = invertTree(root->right); root->right = invertTree(tmp); return root; } };
非遞歸的方法也不復雜,跟二叉樹的層序遍歷同樣,須要用queue來輔助,先把根節點排入隊列中,而後從隊中取出來,交換其左右節點,若是存在則分別將左右節點在排入隊列中,以此類推直到隊列中木有節點了中止循環,返回root便可。代碼以下:url
// Non-Recursion class Solution { public: TreeNode* invertTree(TreeNode* root) { if (!root) return NULL; queue<TreeNode*> q; q.push(root); while (!q.empty()) { TreeNode *node = q.front(); q.pop(); TreeNode *tmp = node->left; node->left = node->right; node->right = tmp; if (node->left) q.push(node->left); if (node->right) q.push(node->right); } return root; } };