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关键字：字符串计数、字符串分割枚举

package dailyexecise;

import java.util.HashMap;
import java.util.Map;

public class numOfPairs2023 {

    public static void main(String[] args) {

        String test = "12345";
        System.out.println(test.substring(0, 2));
        System.out.println(test.substring(3));

    }

    public int numOfPairs(String[] nums, String target) {
        int count = 0;
        for (int i = 0; i < nums.length; i++) {
            for (int j = 0; j < nums.length; j++) {
                String resStr = nums[i] + nums[j];
                if (i != j && target.equals(resStr)) {
                    count++;
                }
            }
        }
        return count;
    }

    public int numOfPairs2(String[] nums, String target) {
        int count = 0;
        for (int i = 0; i < nums.length - 1; i++) {
            // 题目要求需满足 i != j
            for (int j = i+1; j < nums.length; j++) {
                String resStr = nums[i] + nums[j];
                if (target.equals(nums[i] + nums[j])) {
                    count++;
                }
                if (target.equals(nums[j] + nums[i])) {
                    count++;
                }
            }
        }
        return count;
    }

    public int numOfPairs3(String[] nums, String target) {
        // 字符数组计数
        Map<String, Integer> countMap = new HashMap<>();
        for (String num : nums) {
            // key ,insert-value,merge-function
            countMap.merge(num, 1, (old, newValue) -> old + 1);
        }
        int ans = 0;
        // 分解字符串，获取字符串枚举方案
        for (int i = 1; i < target.length(); i++) {
            // substring左闭右开
            String left = target.substring(0, i);
            String right = target.substring(i);
            if (left.equals(right)) {
                // 想不清楚就模拟咯
                ans += countMap.getOrDefault(left, 0) * (countMap.getOrDefault(right, 0) - 1);
            } else {
                ans += countMap.getOrDefault(right, 0) * countMap.getOrDefault(left, 0);
            }
        }
        return ans;
    }

}