LeetCode 94. 二叉树的中序遍历

 迭代

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
       vector<int> ans;
        if(!root) return ans;
        stack<TreeNode*> nodeStack;
        while(root) 
        {
            nodeStack.push(root);
            root = root->left;
        }
        while(!nodeStack.empty()) {
            TreeNode* node = nodeStack.top();
            ans.push_back(node->val);
            nodeStack.pop();
            if(node = node->right) {
                while(node) {
                    nodeStack.push(node);
                    node = node->left;
                } 
            }
        }

        return ans;
    }
};

优化：

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode*> st; 
        while (root!=NULL || !st.empty()) {
            while (root != NULL) {
                st.push(root);
                root = root->left;
            }
            root = st.top();
            st.pop();
            res.push_back(root->val);
            root = root->right;
        }
        return res;
    }
};

递归：

class Solution {
public:
     void process(TreeNode* root,vector<int> &ans)
    {
      
        if(root->left) process(root->left,ans);
          ans.push_back(root->val);
        if(root->right) process(root->right,ans);
    }

    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> ans;
        if(root)
            process(root,ans);
        return ans;
    }
};