454. 四数相加 II (学会了,二刷的我,应该也会)

class Solution {
    public int fourSumCount(int[] nums1, int[] nums2, int[] nums3, int[] nums4) {
//      Map< num1+num2 , 出现的次数 >
        Map<Integer, Integer> map1 = new HashMap<>();
        int tmp;
        for (int num1 : nums1) {
            for (int num2 : nums2) {
                tmp = num1 + num2;
                if (!map1.containsKey(tmp)) {
                    map1.put(tmp, 1);
                } else {
                    map1.put(tmp, map1.get(tmp) + 1);
                }
            }
        }
        int count = 0;
        for (int num3 : nums3) {
            for (int num4 : nums4) {
                if (map1.containsKey(-(num3 + num4))) {
                    count += map1.get(-(num3 + num4));
                }
            }
        }
        return count;
    }
}

383. 赎金信 (ez)

class Solution {
    public boolean canConstruct(String ransomNote, String magazine) {
int[] count = new int[26];
        char ch;
        for (int i = 0; i < magazine.length(); i++) {
            ch = magazine.charAt(i);
            count[ch - 'a'] += 1;
        }
        for (int i = 0; i < ransomNote.length(); i++) {
            ch = ransomNote.charAt(i);
            int tmp = count[ch - 'a'] - 1;
            if (tmp < 0) {
                return false;
            }
            count[ch - 'a'] = tmp;
        }
        return true;
    }
}

下面这两个浓眉大眼的 学...会了吗?

15. 三数之和 (学...会了吧!)

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
List<List<Integer>> answer = new ArrayList<>();
        List<Integer> tmp = new ArrayList<>();
        Arrays.sort(nums);
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] > 0) {
                return answer;
            }
            if (i > 0 && nums[i] == nums[i - 1]) {  // 去重a
                continue;
            }

            int right = nums.length - 1, left = i + 1;
            while (right > left) {
                int sum = nums[i] + nums[left] + nums[right];
                if (sum > 0) {
                    right--;
                } else if (sum < 0) {
                    left++;
                } else {
                    answer.add(Arrays.asList(nums[i], nums[left], nums[right]));
                    while (right > left && nums[right - 1] == nums[right]) right--;
                    while (right > left && nums[left + 1] == nums[left]) left++;
                    right--;
                    left++;
                }
            }
        }
        return answer;
    }
}

18. 四数之和 (学...会了吗?)

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> answer = new ArrayList<>();
        Arrays.sort(nums);
        for (int k = 0; k < nums.length; k++) {
            if (nums[k] > target && nums[k] > 0) {
                return answer;
            }
            if (k > 0 && nums[k] == nums[k - 1]) { // 去重k
                continue;
            }
            for (int i = k + 1; i < nums.length; i++) {
                if (i > k + 1 && nums[i - 1] == nums[i]) {
                    continue;
                }
                int right = nums.length - 1, left = i + 1;
                while (right > left) {
                    long sum = nums[k] + nums[i] + nums[left] + nums[right];
                    if (sum > target) {
                        right--;
                    } else if (sum < target) {
                        left++;
                    } else {
                        answer.add(Arrays.asList(nums[k], nums[i], nums[left], nums[right]));
                        while (right > left && nums[right] == nums[right - 1]) right--;
                        while (right > left && nums[left] == nums[left + 1]) left++;

                        right--;
                        left++;
                    }
                }

            }
        }
        return answer;
    }
}