关于数组以及数字

class Solution {
    public static List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> ans = new ArrayList();
        int len = nums.length;
        if(nums == null || len < 3) return ans;
        Arrays.sort(nums); // 排序
        for (int i = 0; i < len ; i++) {
            if(nums[i] > 0) break; // 如果当前数字大于0，则三数之和一定大于0，所以结束循环
            if(i > 0 && nums[i] == nums[i-1]) continue; // 去重
            int L = i+1;
            int R = len-1;
            while(L < R){
                int sum = nums[i] + nums[L] + nums[R];
                if(sum == 0){
                    ans.add(Arrays.asList(nums[i],nums[L],nums[R]));
                    while (L<R && nums[L] == nums[L+1]) L++; // 去重
                    while (L<R && nums[R] == nums[R-1]) R--; // 去重
                    L++;
                    R--;
                }
                else if (sum < 0) L++;
                else if (sum > 0) R--;
            }
        }        
        return ans;
    }
}

class Solution {
public:
    string intToRoman(int num)
    {
        char* c[4][10] = {
            {"","I","II","III","IV","V","VI","VII","VIII","IX"},
            {"","X","XX","XXX","XL","L","LX","LXX","LXXX","XC"},
            {"","C","CC","CCC","CD","D","DC","DCC","DCCC","CM"},
            {"","M","MM","MMM"}
        };
        string roman;
        roman.append(c[3][num / 1000]);
        roman.append(c[2][num / 100 % 10]);
        roman.append(c[1][num / 10 % 10]);
        roman.append(c[0][num % 10]);
         
        return roman;
    }
};

class Solution {
    public int romanToInt(String s) {
        Map<String, Integer> map = new HashMap<>();
        map.put("I", 1);
        map.put("IV", 4);
        map.put("V", 5);
        map.put("IX", 9);
        map.put("X", 10);
        map.put("XL", 40);
        map.put("L", 50);
        map.put("XC", 90);
        map.put("C", 100);
        map.put("CD", 400);
        map.put("D", 500);
        map.put("CM", 900);
        map.put("M", 1000);
        
        int ans = 0;
        for(int i = 0;i < s.length();) {
            if(i + 1 < s.length() && map.containsKey(s.substring(i, i+2))) {
                ans += map.get(s.substring(i, i+2));
                i += 2;
            } else {
                ans += map.get(s.substring(i, i+1));
                i ++;
            }
        }
        return ans;
    }
}

class Solution {
    public int threeSumClosest(int[] nums, int target) {
        Arrays.sort(nums);
        int ans = nums[0] + nums[1] + nums[2];
        for(int i=0;i<nums.length;i++) {
            int start = i+1, end = nums.length - 1;
            while(start < end) {
                int sum = nums[start] + nums[end] + nums[i];
                if(Math.abs(target - sum) < Math.abs(target - ans))
                    ans = sum;
                if(sum > target)
                    end--;
                else if(sum < target)
                    start++;
                else
                    return ans;
            }
        }
        return ans;
    }
}