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Leetcode: Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes‘ values.

For example:
Given binary tree {1,#,2,3},

   1         2    /   3

 

return [1,3,2].

Note: Recursive solution is trivial, could you do it iteratively?

分析:迭代版inorder traversal。对于任何一棵树,其最先被访问的是路径是最左侧的路径,在该最左侧路径上访问顺序呢是从叶子结点到根节点,很自然我们可以用一个stack保存这个路径。同时如果当前访问结点有右孩子,我们需把以右孩子为根的子树最左路径压入栈中。时间复杂度为O(n),空间复杂度为O(h)。代码如下:

 1 class Solution { 2 public: 3     vector<int> inorderTraversal(TreeNode *root) { 4         vector<int> result; 5         if(!root) return result; 6          7         stack<TreeNode*> S; 8         for(TreeNode *p = root; p; p = p->left) 9             S.push(p);10         11         while(!S.empty()){12             TreeNode *tmp = S.top();13             S.pop();14             result.push_back(tmp->val);15             for(TreeNode *p = tmp->right; p ; p = p->left)16                 S.push(p);17         }18         19         return result;20     }21 };

 

Leetcode: Binary Tree Inorder Traversal