hello-algo/codes/cpp/chapter_backtracking/preorder_traversal_iii_template.cpp

/**
 * File: preorder_traversal_iii_template.cpp
 * Created Time: 2023-04-16
 * Author: Krahets (krahets@163.com)
 */

#include "../utils/common.hpp"

/* 判断当前状态是否为解 */
bool isSolution(vector<TreeNode *> &state) {
    return !state.empty() && state.back()->val == 7;
}

/* 记录解 */
void recordSolution(vector<TreeNode *> &state, vector<vector<TreeNode *>> &res) {
    res.push_back(state);
}

/* 判断在当前状态下，该选择是否合法 */
bool isValid(vector<TreeNode *> &state, TreeNode *choice) {
    return choice != nullptr && choice->val != 3;
}

/* 更新状态 */
void makeChoice(vector<TreeNode *> &state, TreeNode *choice) {
    state.push_back(choice);
}

/* 恢复状态 */
void undoChoice(vector<TreeNode *> &state, TreeNode *choice) {
    state.pop_back();
}

/* 回溯算法：例题三 */
void backtrack(vector<TreeNode *> &state, vector<TreeNode *> &choices, vector<vector<TreeNode *>> &res) {
    // 检查是否为解
    if (isSolution(state)) {
        // 记录解
        recordSolution(state, res);
        return;
    }
    // 遍历所有选择
    for (TreeNode *choice : choices) {
        // 剪枝：检查选择是否合法
        if (isValid(state, choice)) {
            // 尝试：做出选择，更新状态
            makeChoice(state, choice);
            // 进行下一轮选择
            vector<TreeNode *> nextChoices{choice->left, choice->right};
            backtrack(state, nextChoices, res);
            // 回退：撤销选择，恢复到之前的状态
            undoChoice(state, choice);
        }
    }
}

/* Driver Code */
int main() {
    TreeNode *root = vecToTree(vector<int>{1, 7, 3, 4, 5, 6, 7});
    cout << "\n初始化二叉树" << endl;
    printTree(root);

    // 回溯算法
    vector<TreeNode *> state;
    vector<TreeNode *> choices = {root};
    vector<vector<TreeNode *>> res;
    backtrack(state, choices, res);

    cout << "\n输出所有根节点到节点 7 的路径，要求路径中不包含值为 3 的节点" << endl;
    for (vector<TreeNode *> &path : res) {
        vector<int> vals;
        for (TreeNode *node : path) {
            vals.push_back(node->val);
        }
        printVector(vals);
    }
}