【leetcode】剑指 Offer 07. 重建二叉树

 

//每次找到中间节点 再左右分治
struct TreeNode* buildTree(int* preorder, int preorderSize, int* inorder, int inorderSize){
    if(preorderSize==0)
        return NULL;
    struct TreeNode *root=(struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->val=preorder[0];
    int i;
    for(i=0;i<inorderSize;i++)
        if(inorder[i]==preorder[0])
            break;
    root->left=buildTree(&preorder[1],i,&inorder[0],i);
    root->right=buildTree(&preorder[i+1],preorderSize-i-1,&inorder[i+1],preorderSize-i-1);
    return root;
}

 

//C语言栈迭代算法
struct TreeNode* buildTree(int* preorder, int preorderSize, int* inorder, int inorderSize){
    if (preorderSize == 0 || inorderSize == 0)
        return NULL;
    struct TreeNode* stack[10000];
    int top = -1, i, j = 0;
    struct TreeNode* root = (struct TreeNode*)calloc(sizeof(struct TreeNode), 1);
    root->val = preorder[0];
    stack[++top] = root;
    struct TreeNode* temp = root;
    for (i = 1; i<preorderSize; i++){
        struct TreeNode* node = (struct TreeNode*)calloc(sizeof(struct TreeNode), 1);
        if (top >= 0 && j < inorderSize && stack[top]->val == inorder[j]){
            while (i<preorderSize && top >= 0 && j < inorderSize && stack[top]->val == inorder[j]){
                node = stack[top--];
                j++;
            }
            stack[++top] = (struct TreeNode*)calloc(sizeof(struct TreeNode), 1);
            stack[top]->val = preorder[i];
            node->right = stack[top];
        }
        else{
            node->val = preorder[i];
            temp->left = node;
            stack[++top] = node;
        }
        temp = stack[top];
    }
    return root;
}

 

 

struct TreeNode* recursion(int* preorder,int preorderSize,int* inorder,int inorderSize,int* preIndex, int* inIndex,int* hash){
    if((*preIndex)>=preorderSize || (*inIndex)>=inorderSize)
        return NULL;
    struct TreeNode* node = (struct TreeNode*)calloc(sizeof(struct TreeNode),1);
    node->val=preorder[(*preIndex)];
    hash[preorder[(*preIndex)]+20000]++;
    if(preorder[(*preIndex)]==inorder[(*inIndex)]){
        (*inIndex)++;
        if((*inIndex)<inorderSize && (*preIndex)<preorderSize && hash[inorder[(*inIndex)]+20000]){
            (*preIndex)++;
            (*inIndex)++;
            return node;
        }
        (*preIndex)++;
        node->right=recursion(preorder,preorderSize,inorder,inorderSize,preIndex,inIndex,hash);
        return node;                  
    }
    (*preIndex)++;
    node->left=recursion(preorder,preorderSize,inorder,inorderSize,preIndex,inIndex,hash);
    if((*inIndex)<inorderSize && hash[inorder[(*inIndex)]+20000]){
        (*inIndex)++;
        return node;
    }        
    node->right=recursion(preorder,preorderSize,inorder,inorderSize,preIndex,inIndex,hash);    
    return node;
}
struct TreeNode* buildTree(int* preorder, int preorderSize, int* inorder, int inorderSize){
    if (preorderSize==0 || inorderSize==0) return NULL;
    int preIndex=0, inIndex=0;
    int hash[40001]={0};
    return recursion(preorder,preorderSize,inorder,inorderSize,&preIndex,&inIndex,hash);
}