指定k个整数组合(dfs+回溯)

Author: cls1991

Backtracking/216_CombinationSumIII.py

@@ -0,0 +1,58 @@
+# coding: utf8
+
+
+"""
+    题目链接: https://leetcode.com/problems/combination-sum-iii/description.
+    题目描述:
+
+    Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be
+    used and each combination should be a unique set of numbers.
+
+
+    Example 1:
+
+    Input: k = 3, n = 7
+
+    Output:
+
+    [[1,2,4]]
+
+    Example 2:
+
+    Input: k = 3, n = 9
+
+    Output:
+
+    [[1,2,6], [1,3,5], [2,3,4]]
+    Credits:
+    Special thanks to @mithmatt for adding this problem and creating all test cases.
+
+"""
+
+
+class Solution(object):
+    def combinationSum3(self, k, n):
+        """
+        :type k: int
+        :type n: int
+        :rtype: List[List[int]]
+        """
+        if n < k:
+            return []
+
+        ans = []
+        sub = []
+        self.backtrace(k, n, 1, sub, ans)
+
+        return ans
+
+    def backtrace(self, k, n, idx, sub, ans):
+        if n < 0 or len(sub) > k:
+            return
+        elif n == 0 and len(sub) == k:
+            ans.append(sub[:])
+        else:
+            for i in range(idx, 10):
+                sub.append(i)
+                self.backtrace(k, n - i, i + 1, sub, ans)
+                sub.pop()