二叉树（二）

二叉树的其他操作

利用遍历创建二叉树

前序遍历和中序遍历创建二叉树

TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder){
return buildT(preorder,inorder,0,preorder.size()-1,0,inorder.size()-1);
}
TreeNode* build(vector<int>& preorder, vector<int>& inorder, int ps,int pe, int is, int ie){
if(ps>pe) return NULL;
TreeNode* node=new TreeNode(preorder[ps]);
int pos=0;
for(int i=is;i<=ie;i++){
if(inorder[i]==preorder[ps]){
pos=i;
break;
}
}
node->left=buildT(preorder,inorder,ps+1,ps+is-pos;is,pos-1);
node->right=buildT(preorder,inorder,pe-ie+pos+1,pe,pos+1,ie);
return node;
}

中序遍历和后序遍历创建二叉树

TreeNode *buildTree(vector<int> &inorder, vector<int> &postorder) {
return buildTree(inorder, postorder, 0, inorder.size()-1,  0, postorder.size()-1);
}
TreeNode *buildTree(vector<int> &inorder, vector<int> &postorder, int is, int ie, int ps, int pe) {
if(is>ie || ps>pe) return NULL;
TreeNode* node=new TreeNode(postorder[pe]);
int pos=0;
for(int i=is;i<=ie;i++){
if(inorder[i]==postorder[pe]){
pos=i;
break;
}
}
node->left=buildTree(inorder,postorder,is,pos-1,ps,ps+pos-is-1);
node->right=buildTree(inorder,postorder,pos+1,ie,pe-ie+pos,pe-1);
return node;
}

最低公共祖先

BT

TreeNode *LCA(TreeNode*root,TreeNode*p,TreeNode*q){
if(root==NULL || root==p||root==q)
return root;
TreeNode* left=LCA(root->left,p,q);
TreeNode* right=LCA(root->right,p,q);

if(left&&right) return root;
return left?left:right;
}

BST

TreeNode * LCAofBST(TreeNode* root,TreeNode* p,TreeNode* q){
if(root->val>q->val && root->val>p->val)
return LCAofBST(root->left,p,q);
else if(root->val<q->val && root->val<p->val)
return LCAofBST(root->right,p,q);
else return root;
}

将二叉树拆成单链表

void BTtoList(TreeNode* root){
while(root){
if(root->right && root->left){
TreeNode* temp=root->left;
while(temp->right){
temp=temp->right;
}
temp->right=root->right;
}
if(root->left)
root->right=root->left;
root->left=NULL;
root=root->right;
}
}

子树

bool isSubtree(TreeNode* T1,TreeNode* T2){
if(T2==NULL) return false;
if(T1==NULL) return true;
return isIdentical(T1,T2) || isSubtree(T1->left,T2) || isSubtree(T1->right,T2);
}
bool isIdentical(TreeNode* T1,TreeNode* T2){
if(T1==NULL && T2==NULL) return true;
if(T1==NULL || T2==NULL) return false;
if(T1->val!=T2->val) return false;
return isIdentical(T1->left,T2->left) && isIdentical(T1->right,T1->right);
}

二叉树的序列化和反序列化

string serializeBT(TreeNode* root){
ostringstream out;
serial(root,out);
return out.str();
}
TreeNode* deserializeBT(string s){
istringstream in(s);
return deserial(s)
}
void serial(TreeNode* root, ostringstream &out){
if(root){
out<<root->val<<' ';
serial(root->left,out);
serial(root->right,out);
}
else out<<"# ";
}
TreeNode* deserial(istringstream &in){
string s;
in>>s;
if(s=="#") return NULL;
TreeNode *root=new TreeNode(stoi(s));
root->left=deserial(in);
root->right=deserial(in);
return root;
}