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74 lines
1.8 KiB
74 lines
1.8 KiB
1 year ago
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/**
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* File: preorder_traversal_iii_template.dart
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* Created Time: 2023-08-10
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* Author: liuyuxin (gvenusleo@gmail.com)
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*/
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import '../utils/print_util.dart';
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import '../utils/tree_node.dart';
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/* 判断当前状态是否为解 */
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bool isSolution(List<TreeNode> state) {
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return state.isNotEmpty && state.last.val == 7;
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}
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/* 记录解 */
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void recordSolution(List<TreeNode> state, List<List<TreeNode>> res) {
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res.add(List.from(state));
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}
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/* 判断在当前状态下,该选择是否合法 */
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bool isValid(List<TreeNode> state, TreeNode? choice) {
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return choice != null && choice.val != 3;
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}
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/* 更新状态 */
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void makeChoice(List<TreeNode> state, TreeNode? choice) {
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state.add(choice!);
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}
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/* 恢复状态 */
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void undoChoice(List<TreeNode> state, TreeNode? choice) {
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state.removeLast();
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}
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/* 回溯算法:例题三 */
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void backtrack(
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List<TreeNode> state,
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List<TreeNode?> choices,
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List<List<TreeNode>> res,
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) {
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// 检查是否为解
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if (isSolution(state)) {
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// 记录解
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recordSolution(state, res);
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}
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// 遍历所有选择
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for (TreeNode? choice in choices) {
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// 剪枝:检查选择是否合法
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if (isValid(state, choice)) {
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// 尝试:做出选择,更新状态
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makeChoice(state, choice);
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// 进行下一轮选择
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backtrack(state, [choice!.left, choice.right], res);
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// 回退:撤销选择,恢复到之前的状态
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undoChoice(state, choice);
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}
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}
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}
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/* Driver Code */
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void main() {
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TreeNode? root = listToTree([1, 7, 3, 4, 5, 6, 7]);
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print("\n初始化二叉树");
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printTree(root);
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// 回溯算法
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List<List<TreeNode>> res = [];
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backtrack([], [root!], res);
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print("\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");
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for (List<TreeNode> path in res) {
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print(List.from(path.map((e) => e.val)));
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}
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}
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