学步园

void KSum(vector<int> &num, vector<int> &ans, int start, int k, int target) {
        int len = (int)num.size();
        if (k == 2) {
            int L = start, R = len-1;
            unordered_set<int> hasher;
            while (L < R) {
                if (target == num[L] + num[R]) {
                    ans[K-2] = num[L];
                    ans[K-1] = num[R];
                    if (hasher.find(num[L]) == hasher.end()) {
                        result.push_back(ans);
                        hasher.insert(num[L]);
                    }
                    L++; R--;
                }
                else if (target < num[L] + num[R]) R--;
                else L++;
            }
        }
        else {
            for (int i = start; i < len-2; i++) {
                if (i>start && num[i]==num[i-1]) continue;
                ans[K-k] = num[i];
                KSum(num, ans, i+1, k-1, target-num[i]);
            }
        }
    }

递归过程

分解子问题一直到2Sum，这个用两个指针就可以解决了

【最近手残好难过，正确率好低】