update problem

Author: billryan

zh-hans/exhaustive_search/combination_sum.md

@@ -1,33 +1,50 @@
 # Combination Sum
 
+Tags: Array, Backtracking, Medium
+
 ## Question
 
-- leetcode: [Combination Sum | LeetCode OJ](https://leetcode.com/problems/combination-sum/)
-- lintcode: [(135) Combination Sum](http://www.lintcode.com/en/problem/combination-sum/)
+- leetcode: [Combination Sum](https://leetcode.com/problems/combination-sum/)
+- lintcode: [Combination Sum](https://www.lintcode.com/problem/combination-sum/)
+
+### Problem Statement
+
+Given a **set** of candidate numbers (`candidates`) **(without duplicates)**
+and a target number (`target`), find all unique combinations in `candidates`
+where the candidate numbers sums to `target`.
+
+The **same** repeated number may be chosen from `candidates` unlimited number
+of times.
+
+**Note:**
+
+  * All numbers (including `target`) will be positive integers.
+  * The solution set must not contain duplicate combinations.
+
+**Example 1:**
+    
+    
+    
+    **Input:** candidates = [2,3,6,7], target = 7,
+    **A solution set is:**
+    [
+      [7],
+      [2,2,3]
+    ]
+    
+
+**Example 2:**
+    
+    
+    
+    **Input:** candidates = [2,3,5], target = 8,
+    **A solution set is:**
+    [
+      [2,2,2,2],
+      [2,3,3],
+      [3,5]
+    ]
 
-```
-Given a set of candidate numbers (C) and a target number (T),
-find all unique combinations in C where the candidate numbers sums to T.
-The same repeated number may be chosen from C unlimited number of times.
-
-For example, given candidate set 2,3,6,7 and target 7,
-A solution set is:
-[7]
-[2, 2, 3]
-
-Have you met this question in a real interview? Yes
-Example
-given candidate set 2,3,6,7 and target 7,
-A solution set is:
-[7]
-[2, 2, 3]
-
-Note
-- All numbers (including target) will be positive integers.
-- Elements in a combination (a1, a2, … , ak) must be in non-descending order.
-(ie, a1 ≤ a2 ≤ … ≤ ak).
-- The solution set must not contain duplicate combinations.
-```
 
 ## 题解