四数之和python递归写法

1、正确python3递归写法

from typing import List
class Solution:
    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
        def nSumtarget(nums,n,start,target):
            sz = len(nums)
            res=[]
            if n<2 or sz<n:
                return res
            # 2sum为base case
            if n==2:
                lo = start
                hi = sz-1
                while (lo<hi):
                    
                    sum = nums[lo]+nums[hi]
                    left = nums[lo]
                    right = nums[hi]
                    if sum<target:
                        while lo<hi and nums[lo]==left:
                            lo+=1
                    elif sum>target:
                        while lo<hi and nums[hi]==right:
                            hi-=1
                    else:
                        res.append([left,right])
                        while lo<hi and nums[lo]==left:
                            lo+=1
                        while lo<hi and nums[hi]==right:
                            hi-=1
                
            else:
                #n>2时，递归计算(n-1)sum的结果
                #for i in range(start,sz):
                i=start
                while i<sz:
                    print(i) #若第46行不存在，会陷入死循环
                    sub = nSumtarget(nums,n-1,i+1,target-nums[i])
                    #print("返回:%d"%i)
                    for arr in sub:
                        #(n-1)sum加上nums[i]就是nSum
                        arr.append(nums[i])
                        res.append(arr)
                    while (i<sz-1 and nums[i] == nums[i+1]):
                        #print(i)
                        i+=1
                    i=i+1 #注意,这里i值不加1会陷入死循环(i+=1的目的是实现外层循环)
            return res
        
        nums.sort()
        print(nums)
        return nSumtarget(nums,4,0,target)

if __name__=="__main__":
    nums = [1,0,-1,0,-2,2]
    target = 0
    res = Solution().fourSum(nums,0)
    print(res)

①运行结果(注意while循环的输出)

2、错误写法

from typing import List
class Solution:
    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
        def nSumtarget(nums,n,start,target):
            sz = len(nums)
            res=[]
            if n<2 or sz<n:
                return res
            # 2sum为base case
            if n==2:
                lo = start
                hi = sz-1
                while (lo<hi):
                    
                    sum = nums[lo]+nums[hi]
                    left = nums[lo]
                    right = nums[hi]
                    if sum<target:
                        while lo<hi and nums[lo]==left:
                            lo+=1
                    elif sum>target:
                        while lo<hi and nums[hi]==right:
                            hi-=1
                    else:
                        res.append([left,right])
                        while lo<hi and nums[lo]==left:
                            lo+=1
                        while lo<hi and nums[hi]==right:
                            hi-=1
                
            else:
                #n>2时，递归计算(n-1)sum的结果
                #for i in range(start,sz):
                for i in range(start,sz):
                    print(i) #若第46行不存在，会陷入死循环
                    sub = nSumtarget(nums,n-1,i+1,target-nums[i])
                    #print("返回:%d"%i)
                    for arr in sub:
                        #(n-1)sum加上nums[i]就是nSum
                        arr.append(nums[i])
                        res.append(arr)
                    while (i<sz-1 and nums[i] == nums[i+1]):
                        #print(i)
                        i+=1
                    #i=i+1 #注意,这里i值不加1会陷入死循环(i+=1的目的是实现外层循环)
            return res
        
        nums.sort()
        print(nums)
        return nSumtarget(nums,4,0,target)

if __name__=="__main__":
    nums = [1,0,-1,0,-2,2]
    target = 0
    res = Solution().fourSum(nums,0)
    print(res)

②运行结果（for循环和while循环的不同之处）

3、c++写法

c++中for循环和python3中存在差别，c++中下面的for循环变量i值可再循环内部中的while循环中更新，是全局的；而python的for循环变量i不会循环内部更新

using namespace std;
class Solution {
    public:
    vector<vector<int>> fourSum(vector<int>& nums,int target) {
        sort(nums.begin(), nums.end());
        // n 为 3，从 nums[0] 开始计算和为 0 的三元组
        return nSumTarget(nums, 4, 0, target);
    }

    /* 注意：调用这个函数之前一定要先给 nums 排序 */
    // n 填写想求的是几数之和，start 从哪个索引开始计算（一般填 0），target 填想凑出的目标和
    vector<vector<int>> nSumTarget(
            vector<int>& nums, int n, int start, int target) {

        int sz = nums.size();
        vector<vector<int>> res;
        // 至少是 2Sum，且数组大小不应该小于 n
        if (n < 2 || sz < n) return res;
        // 2Sum 是 base case
        if (n == 2) {
            // 双指针那一套 *** 作
            int lo = start, hi = sz - 1;
            while (lo < hi) {
                int sum = nums[lo] + nums[hi];
                int left = nums[lo], right = nums[hi];
                //cout<<"判断左右指针大小\n";
                if (sum < target) {
                    while (lo < hi && nums[lo] == left) lo++;
                } else if (sum > target) {
                    while (lo < hi && nums[hi] == right) hi--;
                } else {
                    res.push_back({left, right});
                    while (lo < hi && nums[lo] == left) lo++;
                    while (lo < hi && nums[hi] == right) hi--;
                }
            }
            //cout<<"退出循环\n";
        } else {
            // n > 2 时，递归计算 (n-1)Sum 的结果
            for (int i = start; i < sz; i++) {
                cout<<i<<endl;
                vector<vector<int>>
                        sub = nSumTarget(nums, n - 1, i + 1, target - nums[i]);
                //cout<<"返回:"<
                for (vector<int>& arr : sub) {
                    // (n-1)Sum 加上 nums[i] 就是 nSum
                    arr.push_back(nums[i]);
                    res.push_back(arr);
                }
                while (i < sz - 1 && nums[i] == nums[i + 1]) {
                    //cout<
                    i++;
                    //cout<
                    //cout<
                    //cout<
                }
            }
        }
        return res;
    }
};

代码来源：https://mp.weixin.qq.com/s/fSyJVvggxHq28a0SdmZm6Q