Commit 361277d

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add q40

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	`1`	`+package 回溯法.q40_组合总和2;`
	`2`	`+`
	`3`	`+import java.util.*;`
	`4`	`+`
	`5`	`+/**`
	`6`	`+ * 回溯法 O(n*log(n))`
	`7`	`+ */`
	`8`	`+class Solution {`
	`9`	`+`
	`10`	`+ public List<List<Integer>> combinationSum2(int[] candidates, int target) {`
	`11`	`+ List<List<Integer>> res = new ArrayList<>();`
	`12`	`+ if (candidates.length == 0) {`
	`13`	`+ return res;`
	`14`	`+ }`
	`15`	`+ Arrays.sort(candidates);`
	`16`	`+ helper(candidates, target, 0, new LinkedList<>(), res);`
	`17`	`+ return res;`
	`18`	`+ }`
	`19`	`+`
	`20`	`+ public void helper(int[] candidates, int target, int start, LinkedList<Integer> stack, List<List<Integer>> res) {`
	`21`	`+ if (start > candidates.length) {`
	`22`	`+ return;`
	`23`	`+ }`
	`24`	`+ if (target == 0 && !stack.isEmpty()) {`
	`25`	`+ List<Integer> item = new ArrayList<>(stack);`
	`26`	`+ res.add(item);`
	`27`	`+ }`
	`28`	`+ HashSet<Integer> set = new HashSet<>();`
	`29`	`+ for (int i = start; i < candidates.length; ++i) {`
	`30`	`+ if (!set.contains(candidates[i]) && target >= candidates[i]) {`
	`31`	`+ stack.push(candidates[i]);`
	`32`	`+ helper(candidates, target - candidates[i], i + 1, stack, res);`
	`33`	`+ stack.pop();`
	`34`	`+ set.add(candidates[i]);`
	`35`	`+ }`
	`36`	`+ }`
	`37`	`+ }`
	`38`	`+}`

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