77:组合

src/main/java/leetcode/editor/cn/Combinations.java

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+//给定两个整数 n 和 k，返回 1 ... n 中所有可能的 k 个数的组合。 
+//
+// 你可以按任何顺序返回答案。 
+//
+// 
+//
+// 示例 1: 
+//
+// 
+//输入: n = 4, k = 2
+//输出:
+//[
+//  [2,4],
+//  [3,4],
+//  [2,3],
+//  [1,2],
+//  [1,3],
+//  [1,4],
+//] 
+//
+// 示例 2: 
+//
+// 
+//输入: n = 1, k = 1
+//输出: [[1]] 
+//
+// 
+//
+// 提示: 
+//
+// 
+// 1 <= n <= 20 
+// 1 <= k <= n 
+// 
+// Related Topics 数组 回溯 
+// 👍 626 👎 0
+
+package leetcode.editor.cn;
+
+import java.util.ArrayList;
+import java.util.List;
+
+//77:组合
+public class Combinations {
+    public static void main(String[] args) {
+        //测试代码
+        Solution solution = new Combinations().new Solution();
+    }
+
+    //力扣代码
+    //leetcode submit region begin(Prohibit modification and deletion)
+    class Solution {
+        List<List<Integer>> list = new ArrayList<>();
+//        public List<List<Integer>> combine(int n, int k) {
+//            int[] nums = new int[n];
+//            for (int i = 0; i < n; i++) {
+//                nums[i] = i + 1;
+//            }
+//            dfs(nums, new ArrayList<>(), k);
+//            return list;
+//        }
+//
+//        private void dfs(int[] nums, List<Integer> temp, int k) {
+//            if (k == temp.size()) {
+//                list.add(new ArrayList<>(temp));
+//                return;
+//            }
+//
+//            for (int num : nums) {
+//                if (temp.size() == 0 || num > temp.get(temp.size() - 1)) {
+//                    temp.add(num);
+//                    dfs(nums, temp, k);
+//                    temp.remove(temp.size() - 1);
+//                }
+//            }
+//        }
+        public List<List<Integer>> combine(int n, int k) {
+            dfs(n, k, 1, new ArrayList<>());
+            return list;
+        }
+
+        public void dfs(int n, int k, int cur, List<Integer> list) {
+            if (k == 0) {
+                this.list.add(new ArrayList<>(list));
+                return;
+            }
+            for (int i = cur; i <= n - k + 1; i++) {
+                list.add(i);
+                dfs(n, k - 1, i + 1, list);
+                list.remove(list.size() - 1);
+            }
+        }
+    }
+//leetcode submit region end(Prohibit modification and deletion)
+
+}

src/main/java/leetcode/editor/cn/Combinations.md

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+<p>给定两个整数 <code>n</code> 和 <code>k</code>，返回 <code>1 ... n</code> 中所有可能的 <code>k</code> 个数的组合。</p>
+
+<p>你可以按任何顺序返回答案。</p>
+
+<p> </p>
+
+<p><strong>示例 1:</strong></p>
+
+<pre>
+<strong>输入:</strong> n = 4, k = 2
+<strong>输出:</strong>
+[
+  [2,4],
+  [3,4],
+  [2,3],
+  [1,2],
+  [1,3],
+  [1,4],
+]</pre>
+
+<p><strong>示例 2:</strong></p>
+
+<pre>
+<strong>输入:</strong> n = 1, k = 1
+<strong>输出: </strong>[[1]]</pre>
+
+<p> </p>
+
+<p><strong>提示:</strong></p>
+
+<ul>
+	<li><code>1 <= n <= 20</code></li>
+	<li><code>1 <= k <= n</code></li>
+</ul>
+<div><div>Related Topics</div><div><li>数组</li><li>回溯</li></div></div>\n<div><li>👍 626</li><li>👎 0</li></div>