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leetcode77.java

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package leetcode;

import java.util.ArrayList;

import java.util.List;

import java.util.Stack;

/*

1...n 中的k个数的全排列

* */

public class leetcode77 {

private List<List<Integer>> res = new ArrayList<>();

/*

pre 用来记录寻找中的组合,长度从1到k

* */

private void findCombinations(int n, int k, int begin, Stack<Integer> pre) {

if (pre.size()==k){

res.add(new ArrayList<>(pre));

}

for (int i = begin; i <=n; i++) {

pre.add(i);

findCombinations(n,k,i+1,pre); //向下递归 begin 为i+1

pre.pop(); //回溯

}

}

public List<List<Integer>> combine(int n, int k) {

if (n <= 0 || k <= 0 || n < k) {

return res;

}

findCombinations(n,k,1,new Stack<>());

return res;

}

public static void main(String[] args) {

System.out.println(new leetcode77().combine(4,2));

}

}