The general reason is that it’s not initialized, it’s running as a random number;
Or it returns a wild pointer, which is common in test cases with empty sets and requires a pointer to be returned. Most of the time, it will be like this without attention:

    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        TreeNode *result;
        if(preorder.size() == 0 && inorder.size() == 0)
            return result;
        else
            return sub_bT(0, preorder.size()-1, 0, inorder.size()-1, preorder, inorder); 

    }

At this time, we often think that a null pointer is returned, but in fact, it is not initialized. What is returned is a wild pointer, which does not know where to point to. However, when we Run Code, no error will be reported and the result is [], so we think that a null pointer is returned.

    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        TreeNode *result(NULL);
        if(preorder.size() == 0 && inorder.size() == 0)
            return result;
        else
            return sub_bT(0, preorder.size()-1, 0, inorder.size()-1, preorder, inorder); 

    }

Read More:

• Error: php56w-common conflicts with php-common-5.3.3-48.el6_ 8.x86_ 64
• Leetcode 832. Flip image
• The sum of the two numbers of leetcode
• The docker runtime container reported an error: error response from daemon: OCI runtime create failed
• LeetCode 23. Merge k Sorted Lists(java)
• Leetcode: 7. Reverse Integer(JAVA)
• LeetCode 332. Reconstruct Itinerary
• Leetcode-234: palindrome linked list
• Leetcode solution 189 Rotate Array Java version
• Leetcode 34 finds the first and last position of an element in the sorted array (medium)
• R-common errors and their possible causes — Notes
• Cause of runtime error on OJ
• Common errors and modification methods of findbug
• 1143 Lowest Common Ancestor
• panic: runtime error: index out of range
• Summary of common runtimeException exceptions
• Yarn: runtime.ContainerExecutionException : launch container failed
• Linux common problems and Solutions
• Common problems and basic concept knowledge of OpenGL
• Common errors and solutions in MapReduce stage