标签：des style blog color 使用 os io strong
Given a binary tree, return the postorder traversal of its nodes‘ values.
For example:
Given binary tree {1,#,2,3},
1
2
/
3
return [3,2,1].
Note: Recursive solution is trivial, could you do it iteratively?
思路：使用tag标记元素的左右子树是否已经输出完毕。若是，则输出该元素并将其出栈；否则，先遍历其左子树再遍历其右子树，同时修改tag值。
1 class StackNode 2 { 3 public: 4 StackNode():tag(true),node(0){} 5 StackNode( short aTag, TreeNode *aNode ):tag(aTag),node(aNode){} 6 short tag; 7 TreeNode *node; 8 }; 9 10 class Solution { 11 public: 12 vector<int> postorderTraversal(TreeNode *root) { 13 vector<int> values; 14 list<StackNode> stack; 15 // root is empty. 16 if( root == 0 ) { return values; } 17 stack.push_back( StackNode( 1, root ) ); 18 19 // main loop. 20 while( !stack.empty() ) { 21 StackNode& sNode = stack.back(); 22 if( sNode.tag == 1 ) { // need to access left child. 23 sNode.tag = 2; 24 if( sNode.node->left ) { 25 stack.push_back( StackNode( 1, sNode.node->left ) ); 26 } 27 } 28 else if( sNode.tag == 2 ) { // need to access right child. 29 sNode.tag = 3; 30 if( sNode.node->right ) { 31 stack.push_back( StackNode( 1, sNode.node->right ) ); 32 } 33 } 34 else { // access current value. 35 values.push_back( sNode.node->val ); 36 stack.pop_back(); 37 } 38 } 39 return values; 40 } 41 };
以下是网上看到的更加简洁的代码：（若上一个输出的元素是当前节点的左儿子或者右儿子，则说明已经完成了对其左右子树的遍历，输出当前节点并将其出栈；否则，将其左右儿子分别入栈。）
1 class Solution { 2 public: 3 vector<int> postorderTraversal(TreeNode *root) { 4 vector<int> ans; 5 list<TreeNode*> node_list; 6 if(root == NULL) return ans; 7 node_list.push_front(root); 8 TreeNode *head = root; 9 while(!node_list.empty()) 10 { 11 TreeNode *cur = node_list.front(); 12 13 if(cur -> right == head || cur -> left == head || ((cur -> right == NULL) && (cur -> left == NULL))) 14 { 15 node_list.pop_front(); 16 ans.push_back(cur -> val); 17 head = cur; 18 } 19 else 20 { 21 if(cur -> right != NULL) node_list.push_front(cur -> right); 22 if(cur -> left != NULL) node_list.push_front(cur -> left); 23 } 24 } 25 } 26 };
Binary Tree Postorder Traversal,布布扣,bubuko.com
Binary Tree Postorder Traversal
标签：des style blog color 使用 os io strong
原文地址：http://www.cnblogs.com/moderate-fish/p/3904087.html
