15. 三数之和c

回溯写了个超时了。这里写得树层去重还是值得借鉴得。

/**
 * Return an array of arrays of size *returnSize.
 * The sizes of the arrays are returned as *returnColumnSizes array.
 * Note: Both returned array and *columnSizes array must be malloced, assume caller calls free().
 */
int temp[3]={0};
void dfs(int** array,int* nums,int numsSize,int* returnSize,int* column,int index,int count,int sum){
    if(count>3 || (index>=numsSize && count!=3)) return;
    if(count==3&&sum!=0) return;
    if(count==3 && sum==0){
        column[*returnSize]=3;
        array[*returnSize]=(int*)malloc(sizeof(int)*3);
        for(int i=0;i<3;i++) array[*returnSize][i]=temp[i];
        (*returnSize)++;
        return;
    }
    if(sum>0 && nums[index]>0) return;
    for(int i=index;i<numsSize;i++){
        if(i==index||(nums[i]!=nums[i-1])){
            temp[count]=nums[i];
            dfs(array,nums,numsSize,returnSize,column,i+1,count+1,sum+nums[i]);
        }
    }
}

int cmp(const void* a,const void* b){
    return *(int*)a-*(int*)b;
}

int** threeSum(int* nums, int numsSize, int* returnSize, int** returnColumnSizes) {
    qsort(nums,numsSize,sizeof(int),cmp);
    int* column=(int*)malloc(sizeof(int)*3500);
    *returnSize=0;
    int** array=(int**)malloc(sizeof(int*)*3500);
    dfs(array,nums,numsSize,returnSize,column,0,0,0);
    *returnColumnSizes=column;
    return array;
}

结果：