使用中序和前序遍历生成树的代码中的错误是什么？

Question

Input is:- inorder-1 6 8 7 preorder-1 6 7 8 The output I'm getting for the input is 8 4 7 6 1 But the correct output is 8 7 6 1 What is the mistake in this code. I have tried too much to debug the code but all my efforts were in vain. Can you sort this out?

int search(int in[], int start, int end, int curr){
        for(int i=start;i<=end;i++){
            if(in[i]==curr)
                return i;
        }
        //return -1;
    }

Node* build(int in[], int pre[], int start, int end){
    static int idx=0;
    
    if(start>end)
        return NULL;
    
    struct Node* node = new Node(pre[idx++]);
    
    if(start==end)
        return node;
        
    int pos=search(in, start, end, node->data);
    node->left=build(in, pre, start, pos-1);
    node->right=build(in, pre, pos+1, end);
    
    return node;
}

Answer 1

Your code builds the tree correctly when only run once with the input you gave:

    int in[] = {1, 6, 8, 7};
    int pre[] = {1, 6, 7, 8};
    Node* tree = build(in, pre, 0, 3);

You did not provide the code that actually outputs the tree in postorder, but if we assume there is no error in that code, there is still the following problem:

You cannot expect a second correct result from calling build , as then the static idx variable will not be reset to 0, and so you get out-of-range array access, leading to undefined behaviour.

To avoid this, use a non-static variable in a wrapper function, and pass that variable by reference to the actual recursive call:

Node* buildhelper(int in[], int pre[], int start, int end, int &idx){
    if(start>end)
        return nullptr; // In C++ don't use NULL here, but nullptr
    
    struct Node* node = new Node(pre[idx++]);
    
    if(start==end)
        return node;
        
    int pos=search(in, start, end, node->data);
    node->left=buildhelper(in, pre, start, pos-1, idx);
    node->right=buildhelper(in, pre, pos+1, end, idx);
    
    return node;
}

Node* build(int in[], int pre[], int size) {
    int idx = 0, start = 0, end = size - 1;

    return buildhelper(in, pre, 0, size - 1, idx);
}

Note that here build has a different signature: it takes a size argument instead of a start and end arguments.

And, finally, even though search would always find a matching index when the input is consistent, you should still cover the case where no match is found and return an int . The compiler should warn about this.