二叉树专题

Posted 2021-03-02 tingxilin

二叉树中序非递归

• 从root开始，一直往左孩子走入栈，走到头
• 倒退回去到有右孩子的点重复上一个步骤，注意，这中间经过的栈扔出去的点，包括最后一个有右孩子的点都要存到ans里面
• 注意判断stack为空

/**
 * 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 {
    vector<int>ans;
public:
    vector<int> inorderTraversal(TreeNode* root) {
        stack<TreeNode*>tree;
        if(root==NULL)
            return ans;
        tree.push(root);
        while(!tree.empty())
        {
            TreeNode*stacktop=tree.top();
            while(stacktop->left!=NULL)
            {
                tree.push(stacktop->left);
                stacktop=tree.top();
            }
            while(!tree.empty() && tree.top()->right==NULL)
            {
                stacktop=tree.top();
                tree.pop();
                ans.push_back(stacktop->val);
            }
            if(tree.empty())
                return ans;
            stacktop=tree.top();
            ans.push_back(stacktop->val);
            tree.pop();
            tree.push(stacktop->right);
        }
        return ans;
    }
};

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