PAT 1151 LCA in a Binary Tree

这道题目仅仅建立一棵树是不够的.需要找到树中每个值对应的中序遍历的顺序下表.这样,每次把当前值的下表和给定的下表a与b的下表进行比较,得到不同的结果

主要思想就是建立索引.

给出代码:

#include <iostream>
#include<vector>
#include<cstdio>
#include<unordered_map>
struct TreeNode{
    int val;
    TreeNode* left;
    TreeNode* right;
};
using namespace std;
int preorder[10005];
int inorder[10005];
unordered_map<int,int> pos;//找到每一个元素对应中序遍历的位置
TreeNode* constructTree(int beg1,int end1,int beg2,int end2){
    if(beg1>end1){
        return nullptr;
    }
    int val=preorder[beg1];
    int index=beg2;
    for (int i = beg2; i <=end2 ; ++i) {
        if(inorder[i]==val){
            index=i;
        }
    }
    auto *root=new TreeNode();
    root->val=val;
    root->left=constructTree(beg1+1,index-beg2+beg1,beg2,index-1);
    root->right=constructTree(index-beg2+beg1+1,end1,index+1,end2);
    return root;
}
void findLCA(TreeNode* root,int a,int b){
    if(root==nullptr){
        return;
    }
   int posA=pos[a];
   int posB=pos[b];
   int val=root->val;
   int posRoot=pos[val];
   if((posRoot>posA&&posRoot<posB)||(posRoot<posA&&posRoot>posB)){
       printf("LCA of %d and %d is %d.\n",a,b,val);
       return;
   }
   if(posRoot==posA||posRoot==posB){
       printf("%d is an ancestor of %d.\n",val,posRoot==posA?b:a);
       return;
   }
   if(posRoot>max(posA,posB)){
       findLCA(root->left,a,b);
       return;
   }
   findLCA(root->right,a,b);
}
int main() {
    int M,N;
    cin>>M>>N;
    for (int i = 1; i <=N ; ++i) {
        cin>>inorder[i];
        pos[inorder[i]]=i;//安排好位置,在找祖先时方便,找祖先就是看在中序遍历的位置
    }
    for (int i = 1; i <= N; ++i) {
        cin>>preorder[i];
    }
    vector<pair<int,int>> vector1;
    for (int i = 0; i < M; ++i) {
        int a,b;
        cin>>a>>b;
        vector1.emplace_back(a,b);
    }
    TreeNode* root=constructTree(1,N,1,N);
    for(auto pair:vector1){
        int a=pair.first;
        int b=pair.second;
        if(pos[a]==0&&pos[b]==0){
            printf("ERROR: %d and %d are not found.\n",a,b);
        }else if(pos[a]==0||pos[b]==0){
            printf("ERROR: %d is not found.\n",pos[a]==0?a:b);
        } else{
           // findLCA(1,a,b,1,N);
           findLCA(root,a,b);
        }
    }
    return 0;
}