LeetCode 145. Binary Tree Postorder Traversal
先自己在纸上画棵树,发现可以用stack实现迭代版的后序遍历,最开始先把root压入栈。另外维护一个栈,记录对应节点出栈的次数,如果是1,表明左右节点已输出,就输出;如果为0,说明左右节点还未压入栈,就把次数加1,且把左右节点压入栈。
1 /** 2 * Definition for a binary tree node. 3 * struct TreeNode { 4 * int val; 5 * TreeNode *left; 6 * TreeNode *right; 7 * TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8 * }; 9 */ 10 class Solution { 11 public: 12 vector<int> postorderTraversal(TreeNode* root) { 13 vector<int> res; 14 if(!root) return res; 15 stack<TreeNode*> stk; 16 stack<int> time; 17 stk.push(root); 18 time.push(0); 19 while(!stk.empty()){ 20 TreeNode* top = stk.top(); 21 int tmp = time.top(); 22 if(tmp == 0){ 23 ++tmp; time.pop(); time.push(tmp); 24 if(top->right) {stk.push(top->right);time.push(0);} 25 if(top->left) {stk.push(top->left);time.push(0);} 26 } 27 else{ 28 res.push_back(top->val); 29 stk.pop();time.pop(); 30 } 31 32 } 33 return res; 34 } 35 };