The recursive solution is trivial and I omit it here.
Iterative Solution using Stack (O(n) time and O(n) space):
1 /** 2 * Definition of TreeNode: 3 * class TreeNode { 4 * public: 5 * int val; 6 * TreeNode *left, *right; 7 * TreeNode(int val) { 8 * this->val = val; 9 * this->left = this->right = NULL; 10 * } 11 * } 12 */ 13 class Solution { 14 /** 15 * @param root: The root of binary tree. 16 * @return: Postorder in vector which contains node values. 17 */ 18 public: 19 vector<int> postorderTraversal(TreeNode *root) { 20 // write your code here 21 vector<int> nodes; 22 TreeNode* node = root; 23 TreeNode* lastNode = NULL; 24 stack<TreeNode*> toVisit; 25 while (node || !toVisit.empty()) { 26 if (node) { 27 toVisit.push(node); 28 node = node -> left; 29 } 30 else { 31 TreeNode* topNode = toVisit.top(); 32 if (topNode -> right && topNode -> right != lastNode) 33 node = topNode -> right; 34 else { 35 nodes.push_back(topNode -> val); 36 lastNode = topNode; 37 toVisit.pop(); 38 } 39 } 40 } 41 return nodes; 42 } 43 };
Another sophisticated solution using Morris Traversal (O(n) time and O(1) space):
1 /** 2 * Definition of TreeNode: 3 * class TreeNode { 4 * public: 5 * int val; 6 * TreeNode *left, *right; 7 * TreeNode(int val) { 8 * this->val = val; 9 * this->left = this->right = NULL; 10 * } 11 * } 12 */ 13 class Solution { 14 /** 15 * @param root: The root of binary tree. 16 * @return: Postorder in vector which contains node values. 17 */ 18 public: 19 vector<int> postorderTraversal(TreeNode *root) { 20 // write your code here 21 vector<int> nodes; 22 TreeNode* dump = new TreeNode(0); 23 dump -> left = root; 24 TreeNode* node = dump; 25 while (node) { 26 if (node -> left) { 27 TreeNode* predecessor = node -> left; 28 while (predecessor -> right && predecessor -> right != node) 29 predecessor = predecessor -> right; 30 if (!(predecessor -> right)) { 31 predecessor -> right = node; 32 node = node -> left; 33 } 34 else { 35 reverseAddNodes(node -> left, predecessor, nodes); 36 predecessor -> right = NULL; 37 node = node -> right; 38 } 39 } 40 else node = node -> right; 41 } 42 return nodes; 43 } 44 private: 45 void reverseNodes(TreeNode* start, TreeNode* end) { 46 TreeNode* x = start; 47 TreeNode* y = start -> right; 48 TreeNode* z; 49 while (x != end) { 50 z = y -> right; 51 y -> right = x; 52 x = y; 53 y = z; 54 } 55 } 56 void reverseAddNodes(TreeNode* start, TreeNode* end, vector<int>& nodes) { 57 reverseNodes(start, end); 58 TreeNode* node = end; 59 while (true) { 60 nodes.push_back(node -> val); 61 if (node == start) break; 62 node = node -> right; 63 } 64 reverseNodes(end, start); 65 } 66 };