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第77题. 组合

题目链接: 组合

回溯 + 剪枝
可以参考这个题解，写的是真好。

题解

剪枝前：25.78%

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class Solution {

    List<Integer> path = new ArrayList<>();
    List<List<Integer>> result = new ArrayList<>();

    public List<List<Integer>> combine(int n, int k) {
        backtracking(n,k,1);
        return result;
    }

    public void backtracking(int n,int k,int startIndex) {
        if(path.size() == k) {
            result.add(new ArrayList<>(path));
            return;
        }
        
        for(int i = startIndex; i <= n; i++) {
            path.add(i);
            backtracking(n,k,i + 1);
            path.remove(path.size() - 1);
        }
    }
}

剪枝过后：99.97%

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class Solution {

    List<Integer> path = new ArrayList<>();
    List<List<Integer>> result = new ArrayList<>();

    public List<List<Integer>> combine(int n, int k) {
        backtracking(n,k,1);
        return result;
    }

    public void backtracking(int n,int k,int startIndex) {
        if(path.size() == k) {
            result.add(new ArrayList<>(path));
            return;
        }
        
        for(int i = startIndex; i <= n - (k - path.size()) + 1; i++) {
            path.add(i);
            backtracking(n,k,i + 1);
            path.remove(path.size() - 1);
        }
    }
}