update: 0040.组合总和II JS版本增加使用used去重

problems/0040.组合总和II.md

@@ -457,6 +457,37 @@ var combinationSum2 = function(candidates, target) {
     }
 };
 ```
+**使用used去重**
+```js
+var combinationSum2 = function(candidates, target) {
+    let res = [];
+    let path = [];
+    let total = 0;
+    const len = candidates.length;
+    candidates.sort((a, b) => a - b);
+    let used = new Array(len).fill(false);
+    const backtracking = (startIndex) => {
+        if (total === target) {
+            res.push([...path]);
+            return;
+        }
+        for(let i = startIndex; i < len && total < target; i++) {
+            const cur = candidates[i];
+            if (cur > target - total || (i > 0 && cur === candidates[i - 1] && !used[i - 1])) continue;
+            path.push(cur);
+            total += cur;
+            used[i] = true;
+            backtracking(i + 1);
+            path.pop();
+            total -= cur;
+            used[i] = false;
+        }
+    }
+    backtracking(0);
+    return res;
+};
+```
+
 ## C 
 
 ```c