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48 lines
1.7 KiB
Java
48 lines
1.7 KiB
Java
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/**
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* File: permutations_ii.java
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* Created Time: 2023-04-24
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* Author: Krahets (krahets@163.com)
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*/
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package chapter_backtracking;
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import java.util.*;
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public class permutations_ii {
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/* 回溯算法:全排列 II */
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public static void backtrack(List<Integer> state, int[] choices, boolean[] selected, List<List<Integer>> res) {
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// 当状态长度等于元素数量时,记录解
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if (state.size() == choices.length) {
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res.add(new ArrayList<Integer>(state));
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return;
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}
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// 遍历所有选择
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Set<Integer> duplicated = new HashSet<Integer>();
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for (int i = 0; i < choices.length; i++) {
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int choice = choices[i];
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// 剪枝:不允许重复选择元素 且 不允许重复选择相等元素
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if (!selected[i] && !duplicated.contains(choice)) {
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// 尝试
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duplicated.add(choice); // 记录选择过的元素值
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selected[i] = true; // 做出选择
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state.add(choice); // 更新状态
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backtrack(state, choices, selected, res);
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// 回退
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selected[i] = false; // 撤销选择
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state.remove(state.size() - 1); // 恢复到之前的状态
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}
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}
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}
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public static void main(String[] args) {
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int[] nums = { 1, 2, 2 };
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List<List<Integer>> res = new ArrayList<List<Integer>>();
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// 回溯算法
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backtrack(new ArrayList<Integer>(), nums, new boolean[nums.length], res);
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System.out.println("输入数组 nums = " + Arrays.toString(nums));
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System.out.println("所有排列 res = " + res);
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}
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}
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