PAT甲级真题 1054 The Dominant Color (20分) C++实现（找众数）

题目

Behind the scenes in the computer’s memory, color is always talked about as a series of 24 bits of information for each pixel. In an image, the color with the largest proportional area is called the dominant color. A strictly dominant color takes more than half of the total area. Now given an image of resolution M by N (for example, 800×600), you are supposed to point out the strictly dominant color.

Input Specification:

Each input file contains one test case. For each case, the first line contains 2 positive numbers: M (<=800) and N (<=600) which are the resolutions of the image. Then N lines follow, each contains M digital colors in the range [0, 224). It is guaranteed that the strictly dominant color exists for each input image. All the numbers in a line are separated by a space.

Output Specification:

For each test case, simply print the dominant color in a line.

Sample Input:

5 3
0 0 255 16777215 24
24 24 0 0 24
24 0 24 24 24

Sample Output:

思路

找众数，简单写法是对每个元素计数，当某个元素出现次数大于等于n/2时即可认定其为众数（题目保证众数存在）。

也可以转化为找中位数，中位数即是众数，平均复杂度能降低一些，但写起来麻烦点。参考：找第k小数和中位数算法——快速排序思想，复杂度O(n) C++实现

代码

#include <iostream>
#include <unordered_map>
using namespace std;

int main(){
    int n, m;
    scanf("%d %d", &n, &m);
    int len = n * m;
    unordered_map<int, int> mmap;
    for (int i=0; i<len; i++){
        int temp;
        scanf("%d", &temp);
        mmap[temp]++;
        if (mmap[temp] >= len/2){
            cout << temp << endl;
            break;
        }
    }
    return 0;
}