The trap I meet with C++

  • 4

    dURING my implementation.

    I make one mistake as follows:

        if(s>e)  return NULL;
        should be
        if(s>e)  return { NULL };

    As we will loop left and right result to combine with the root node.

    Here is the AC implementation

    class Solution {
        vector<TreeNode*> generateTrees(int n) {
            if(n==0)  return vector<TreeNode*>{};
            return help(1, n);
        vector<TreeNode*> help(int s, int e){
            vector<TreeNode*> result;
            if(s>e)  return {NULL};
            for(int i=s; i<=e; i++){
                vector<TreeNode*> left=help(s, i-1);
                vector<TreeNode*> right=help(i+1, e);
                for(auto l:left){
                    for(auto r:right){
                        TreeNode* root=new TreeNode(i);
            return result;

  • 0
    if(s>e) return NULL;

    Just curious,you did not get compile error from it? The return type is a vector so the compiler should be able to detect it.

    One more thing, using nullptr instead of NULL.

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