逆向双指针

lc1 Two Sum

int twoSum(int[] nums, int target) {
    int i = 0, j = nums.length - 1;
    while (i < j) {
        if (nums[i] + nums[j] < target) i++
        else (nums[i] + nums[j] > target) j--;
        else return new int{nums[i], nums[j]};
    }
    
    return new int[0];
}

int twoSum(int[] nums, int target) {
    int n = nums.length;
    Map<Integer, Integer> val2idx = new HashMap<>();
    for (int i = 0; i < nums.length; i++) {
		if (val2idx.containsKey(nums[i])) {
            return new int[]{val2idx.get(nums[i]), i};
        }
        val2idx.put(target - nums[i], i);
    }
    
    return new int[0];
}

349. Intersection of Two Arrays

Given two integer arrays nums1 and nums2, return an array of their intersection. Each element in the result must be unique and you may return the result in any order.

int[] intersection(int[] nums1, int[] nums2) {
    Arrays.sort(nums1);
    Arrays.sort(nums2);

    Set<Integer> intersection = new HashSet<>();

    int i = 0, j = 0;
    while (i < nums1.length && j < nums2.length) {
        if (nums1[i] == nums2[j]) {
            intersection.add(nums1[i++]);
            j++;
        } else if (nums1[i] < nums2[j]) {
            i++;
        } else {
            j++;
        }
    }

    int[] result = new int[intersection.size()];
    int k = 0;
    for (int x: intersection) {
        result[k++] = x;
    }

    // Return the result
    return result;
}