106. Construct Binary Tree from Inorder and Postorder Traversal
2018-01-10 17:00
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106. Construct Binary Tree from Inorder and Postorder Traversal
题目
Given inorder and postorder traversal of a tree, construct the binary tree. Note: You may assume that duplicates do not exist in the tree.
解析
// 106. Construct Binary Tree from Inorder and Postorder Traversal class Solution_106 { public: TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) { //这样消耗内存多些 if (inorder.size()==0||postorder.size()==0||inorder.size()!=postorder.size()) { return NULL; } int len = postorder.size(); TreeNode* root = new TreeNode(postorder[len-1]); //bug: //terminate called after throwing an instance of 'std::bad_alloc' //what() : std::bad_alloc auto pos = find(inorder.begin(), inorder.end(), postorder[len - 1]); vector<int> inorder_l(inorder.begin(),pos); vector<int> inorder_r(pos + 1, inorder.end()); vector<int> postorder_l(postorder.begin(), postorder.begin() + inorder_l.size()); vector<int> postorder_r(postorder.begin()+inorder_l.size(),postorder.end()-1); if (inorder_l.size()>0) { root->left = buildTree(inorder_l, postorder_l); } if (inorder_r.size()>0) { root->right = buildTree(inorder_r, postorder_r); } return root; } public: TreeNode* buildTreeHelper(vector<int>& inorder, int l1, int r1, vector<int>& postorder, int l2, int r2) //在原数组上操作,不需要额外空间 { if (l1>r1||l2>r2) { return NULL; } TreeNode* root = new TreeNode(postorder[r2]); int i = 0; for ( i= l1; i <= r1;i++) // for ( i= 0; i < inorder.size();i++) //递归实现参数要能进入下次递归 { if (inorder[i]==postorder[r2]) { break; } } root->left = buildTreeHelper(inorder,l1,i-1,postorder,l2,l2+(i-1-l1)); //慢慢体会下标的准确性 root->right = buildTreeHelper(inorder, i + 1, r1, postorder, l2 + (i - l1), r2-1); return root; } TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) { if (inorder.size()==0||postorder.size()==0||inorder.size()!=postorder.size()) { return NULL; } return buildTreeHelper(inorder, 0, inorder.size() - 1, postorder,0, postorder.size() - 1); } };
题目来源
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