哈希表|15.三数之和

news2024/10/6 2:29:09

力扣题目链接

int cmp(const void *a,const void *b) {
    return *(int*)a - *(int*)b;
 }
int** threeSum(int* nums, int numsSize, int* returnSize, int** returnColumnSizes) {
    *returnSize =  0;
    if(numsSize < 3)
        return NULL;
    qsort(nums, numsSize, sizeof(int),cmp);
    int **ans = (int **)malloc(sizeof(int *) * numsSize * numsSize);
    *returnColumnSizes = (int *)malloc(sizeof(int) * numsSize * numsSize);
    int i,j,k,sum;

    int indexLeft = 0;
    int indexMiddle = 0;
    int indexRight = 0;

    for(indexLeft = 0; indexLeft < numsSize - 2; indexLeft++)
    {
        if(nums[indexLeft] > 0)
        {
            return ans;
        }
        if(indexLeft > 0 && nums[indexLeft] == nums[indexLeft - 1])
            continue;
        indexMiddle = indexLeft + 1;
        indexRight = numsSize - 1;

        while(indexMiddle < indexRight)
        {
            sum = nums[indexLeft] + nums[indexMiddle] + nums[indexRight];
            if(sum == 0)
            {
                ans[*returnSize] = (int*)malloc(sizeof(int)*3);
                (*returnColumnSizes)[*returnSize] = 3;
                ans[*returnSize][0] = nums[indexLeft];
                ans[*returnSize][1] = nums[indexMiddle];
                ans[*returnSize][2] = nums[indexRight];
                *returnSize += 1;
                while(indexMiddle < indexRight && nums[indexMiddle] == nums[++indexMiddle]);
                while(indexMiddle < indexRight && nums[indexRight] == nums[--indexRight]);
            }
        else if(sum > 0)
        {
            indexRight--;
        }
        else{
            indexMiddle++;
        }




        }
    }
    return ans;
}

c语言，用快排 + 双指针法

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        vector<vector<int>> result;
        sort(nums.begin(), nums.end());
        // 找出a + b + c = 0
        // a = nums[i], b = nums[j], c = -(a + b)
        for (int i = 0; i < nums.size(); i++) {
            // 排序之后如果第一个元素已经大于零，那么不可能凑成三元组
            if (nums[i] > 0) {
                break;
            }
            if (i > 0 && nums[i] == nums[i - 1]) { //三元组元素a去重
                continue;
            }
            unordered_set<int> set;
            for (int j = i + 1; j < nums.size(); j++) {
                if (j > i + 2
                        && nums[j] == nums[j-1]
                        && nums[j-1] == nums[j-2]) { // 三元组元素b去重
                    continue;
                }
                int c = 0 - (nums[i] + nums[j]);
                if (set.find(c) != set.end()) {
                    result.push_back({nums[i], nums[j], c});
                    set.erase(c);// 三元组元素c去重
                } else {
                    set.insert(nums[j]);
                }
            }
        }
        return result;
    }
};

c++哈希表法