题目链接如下：

Online Judge

开始的代码如下：

#include <iostream>
#include <string>
#include <sstream>
#include <vector>
#include <map>
const int INF = 99999999;
// #define debugstd::vector<int> inorder, postorder;
std::string line;
int t, root, minn, key, sum;
std::map<int, int> inloc, left, right;int findRoot(int inLeft, int inRight, int postLeft, int postRight){if (inLeft == inRight){return 0;}int rt = postorder[postRight - 1];int loc = inloc[rt];left[rt] = findRoot(inLeft, loc, postLeft, loc - inLeft + postLeft);right[rt] = findRoot(loc + 1, inRight, loc - inLeft + postLeft, postRight - 1);return rt;
}void dfs(int k){sum += k;if (!left[k] && !right[k]){if (sum < minn){minn = sum;key = k;} else if (sum == minn && key > k){key = k;}sum -= k;return;}if (left[k]){dfs(left[k]);}if (right[k]){dfs(right[k]);}sum -= k;
}int main(){#ifdef debugfreopen("0.txt", "r", stdin);freopen("1.txt", "w", stdout);#endifwhile (getline(std::cin, line)){std::stringstream in(line);while (in >> t){inloc[t] = inorder.size();inorder.push_back(t);}getline(std::cin, line);std::stringstream post(line);while (post >> t){postorder.push_back(t);}root = findRoot(0, inorder.size(), 0, inorder.size());minn = INF;sum = 0;dfs(root);printf("%d\n", key);inorder.clear();postorder.clear();left.clear();right.clear();inloc.clear();}#ifdef debugfclose(stdin);fclose(stdout);#endifreturn 0;
}

后来看别人的代码发现dfs可以省掉，修改后如下：

#include <iostream>
#include <string>
#include <sstream>
#include <vector>
#include <map>
const int INF = 99999999;
// #define debugstd::vector<int> inorder, postorder;
std::string line;
int t, root, minn, key;
std::map<int, int> inloc, left, right;int findRoot(int inLeft, int inRight, int postLeft, int postRight, int tot){if (inLeft == inRight){return 0;}int rt = postorder[postRight - 1];int sum = tot + rt;int loc = inloc[rt];left[rt] = findRoot(inLeft, loc, postLeft, loc - inLeft + postLeft, sum);right[rt] = findRoot(loc + 1, inRight, loc - inLeft + postLeft, postRight - 1, sum);if (!left[rt] && !right[rt]){if (sum < minn || (sum == minn && rt < key)){minn = sum;key = rt;}}return rt;
}int main(){#ifdef debugfreopen("0.txt", "r", stdin);freopen("1.txt", "w", stdout);#endifwhile (getline(std::cin, line)){std::stringstream in(line);while (in >> t){inloc[t] = inorder.size();inorder.push_back(t);}getline(std::cin, line);std::stringstream post(line);while (post >> t){postorder.push_back(t);}minn = INF;root = findRoot(0, inorder.size(), 0, inorder.size(), 0);printf("%d\n", key);inorder.clear();postorder.clear();left.clear();right.clear();inloc.clear();}#ifdef debugfclose(stdin);fclose(stdout);#endifreturn 0;
}