Leetcode 105.从前序遍历与中序遍历序构造二叉树

class Solution {
public:
    unordered_map m;  //构造一个哈希表方便定位inorder数组中根节点的位置
    TreeNode* build(vector& preorder,int preStart,int preEnd,vector& inorder,int inStart,int inEnd){
        if(preStart>preEnd){
            return NULL;
        }
        int rootVal=preorder[preStart];
        int index=m[rootVal];   //index对应inorder数组中根节点的下标
        int left_size=index-inStart;  // 得到左子树中的节点数目
        TreeNode* root=new TreeNode(rootVal);
        root->left=build(preorder,preStart+1,preStart+left_size,inorder,inStart,inEnd-1);
        root->right=build(preorder,preStart + left_size + 1, preEnd,inorder, index + 1, inEnd);
        return root;
    }
    TreeNode* buildTree(vector& preorder, vector& inorder) {
        int n=preorder.size();
        for(int i=0;i