DFS & BFS

DFS : depth first search

BFS: breadth first search

DFS : 

46. Permutations

Given a collection of distinct numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:

[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]

public class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> member = new ArrayList<Integer>();
        if(nums == null || nums.length == 0){
            return res;
        }
        dfs(res, member, nums);
        return res;
    }
    public void dfs(List<List<Integer>> res, List<Integer> member, int[] nums){
        if(member.size()== nums.length){
            res.add(new ArrayList<Integer>(member));
            return;
        }
        for(int i = 0; i < nums.length; i ++){
            if(member.contains(nums[i])) continue;
            member.add(nums[i]);
            dfs(res, member, nums);
            member.remove(member.size()-1);
        }
    }
}

47. Permutations II

Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,
[1,1,2] have the following unique permutations:

[
  [1,1,2],
  [1,2,1],
  [2,1,1]
]

public class Solution {
    public List<List<Integer>> permuteUnique(int[] nums) {
        List<List<Integer>>  res = new ArrayList<>();
        List<Integer> member = new ArrayList<>();
        boolean visited[] = new boolean[nums.length];
        if(nums == null || nums.length == 0){
            return res;
        }
        Arrays.sort(nums);
        dfs(res, member, nums, visited);
        return res;
    }
    public void dfs(List<List<Integer>> res, List<Integer> member, int[] nums, boolean[] visited){
        if(member.size() == nums.length){ 
            if(!res.contains(member)){
                res.add(new ArrayList<>(member));
                return;
            }
        }
        for(int i = 0; i<nums.length; i++){
            if(!visited[i]){
                if(i > 0 && nums[i] == nums[i - 1] && !visited[i -1] )
                    continue;
                visited[i] = true;
                member.add(nums[i]);
                dfs(res, member, nums, visited);
                visited[i] = false;
                member.remove(member.size() - 1);
            }
        }
    }
}

31. Next Permutation（不是 dfs）

Implement next permutation, which rearranges numbers into the lexicographically next greater permutation of numbers.

If such arrangement is not possible, it must rearrange it as the lowest possible order (ie, sorted in ascending order).

The replacement must be in-place, do not allocate extra memory.

Here are some examples. Inputs are in the left-hand column and its corresponding outputs are in the right-hand column.
1,2,3 → 1,3,2
3,2,1 → 1,2,3
1,1,5 → 1,5,1

public class Solution {
    // 四步
    // 1.从后向前找 first 小于second的两个值
    // 2.从后向前找第一个大于first的值 revertnum
    // 3.swap first和 revertnum
    // 4. reverse second到数组末尾
    public void nextPermutation(int[] nums) {
        if(nums == null || nums.length == 0 || nums.length == 1) return;
        int first = 0 ;
        int second = 0;
        int revertNum = 0;
        for(int i = nums.length - 1; i > 0 ; i--){
            if( nums[i - 1] < nums[i] ){
                first = i - 1;
                second = i;
                break;
            }
        }
        for(int i = nums.length - 1; i > 0 ; i--){
            if(nums[i] > nums[first]){
                revertNum = i;
                break;
            }
        }
        swap(first, revertNum, nums);
        reverse(second, nums.length -1, nums);
    }
    
    public void swap(int left, int right, int[] nums){
        int temp = nums[left];
        nums[left] = nums[right];
        nums[right] = temp;
    }
    
    public void reverse(int left, int right, int[] nums){
        while(left < right){
            int temp = nums[left];
            nums[left] = nums[right];
            nums[right] = temp;
            left++;
            right--;
        }
    }
}