剑指offer双指针 57-II.和为s的连续正数序列

Posted 2021-02-28 trevo

双指针

[l,r]的区间和：s = (l + r) * (r - l + 1) / 2
通过利用l和r两个指针，初始l=1,r=2;
   如果s == target，将[l,r]的数组添加到结果res中，l++；
   如果s < target, r++;
   如果s > target, l++;
时间复杂度O(n),空间复杂度O(1)

class Solution {
public:
    vector<vector<int>> findContinuousSequence(int target) {
        vector<vector<int>> res;
        vector<int> v;
        int l = 1, r = 2;
        while(l < r)
        {
            int sum = (l + r) * (r - l + 1) / 2;
            if(sum == target){
                v.clear();
                for(int i = l; i <= r; ++i) v.push_back(i);
                res.push_back(v);
                l++;
            }
            else if(sum < target) r++;
            else l++;
        }
        return res;
        
    }
};

滑动窗口

不适用求和公式
比双指针效率更高些，减少多余的计算；
时间复杂度O(n),空间复杂度O(1)

class Solution {
public:
    vector<vector<int>> findContinuousSequence(int target) {
        vector<vector<int>> res;
        int i = 1;
        int j = 1;
        int sum = 0;
        
        while (i <= target / 2) 
        {
            if (sum < target) 
            {
                // 右边界向右移动
                sum += j;
                j++;
            } else if (sum > target) 
            {
                // 左边界向右移动
                sum -= i;
                i++;
            } else 
            {        
                vector<int> arr;       
                for (int k = i; k < j; k++)  arr.push_back(k);
                res.push_back(arr);
                // 左边界向右移动
                sum -= i;
                i++;
            }
        }
        return res;
        
    }
};