双指针

[Algo] 双指针

1. 接雨水

// 1. 接雨水
// https://leetcode.cn/problems/trapping-rain-water/
int trap1(vector<int>& height) {
    vector<int> arr_l, arr_r;
    arr_l.resize(height.size());
    arr_r.resize(height.size());
    int lmax = INT32_MIN, rmax = INT32_MIN;
    for (int i = 0; i < height.size(); i++)
    {
        if (height[i] > lmax) lmax = height[i];
        arr_l[i] = lmax;
    }
    for (int i = height.size() - 1; i >= 0; i--)
    {
        if (height[i] > rmax) rmax = height[i];
        arr_r[i] = rmax;
    }
    int ans = 0;
    for (int i = 1; i < height.size() - 1; i++)
    {
        ans += max(min(arr_l[i - 1], arr_r[i + 1]) - height[i], 0);
    }
    return ans;
}
int trap2(vector<int>& height) {
    int len = height.size();
    int l = 1, r = len - 2, lmax = height[0], rmax = height[len - 1];
    int ans = 0;
    while (l <= r)
    {
        if (lmax <= rmax)
        {
            ans += max(lmax - height[l], 0);
            lmax = max(lmax, height[l++]);
        }
        else
        {
            ans += max(rmax - height[r], 0);
            rmax = max(rmax, height[r--]);
        }
    }
    return ans;
}

2. 救生艇

// 2. 救生艇
// https://leetcode.cn/problems/boats-to-save-people/
int numRescueBoats(vector<int>& people, int limit) {
    int ans = 0, l = 0, r = people.size() - 1, sum;
    sort(people.begin(), people.end());
    while (l <= r)
    {
        sum = l == r ? people[l] : people[l] + people[r];
        if (sum > limit) { r--; ans++; }
        else { l++; r--; ans++; }
    }
    return ans;
}

3. 供暖器

bool best(vector<int>& houses, vector<int>& heaters, int i, int j);
// 3. 供暖器
// https://leetcode.cn/problems/heaters/description/
int findRadius(vector<int>& houses, vector<int>& heaters) {
    sort(houses.begin(), houses.end());
    sort(heaters.begin(), heaters.end());
    int m = houses.size(), n = heaters.size();
    int ans = 0;
    for (int i = 0, j = 0; i < m; i++)
    {
        while (!best(houses, heaters, i, j)) j++;
        ans = max(ans, abs(houses[i] - heaters[j]));
    }
    return ans;
}
bool best(vector<int>& houses, vector<int>& heaters, int i, int j)
{
    return (j == heaters.size() - 1) || (abs(houses[i] - heaters[j]) < abs(houses[i] - heaters[j + 1])); 
}

4. 缺失的第一个正数

// 4. 缺失的第一个正数
// https://leetcode.cn/problems/first-missing-positive/
int firstMissingPositive(vector<int>& nums) {
    int l = 0, r = nums.size();
    // r 最好状态下,1...r能收集全,[r,...]为垃圾区
    while (l < r)
    {
        if (nums[l] == l + 1) l++;
        else if (nums[l] <= l || nums[l] > r || nums[nums[l] - 1] == nums[l]) swap(nums[l], nums[--r]);
        // 移入垃圾区
        else swap(nums[nums[l] - 1], nums[l]);
        // 交换到该放的位置
    }
    return l + 1;
}