必背算法

Posted on | In

基础算法

二分查找

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int binarySearch_leftopen_rightclose_duplicate_uper(vector<int> v, int x){
    int n = v.size();
    if (n < 2) {
        return 0;
    }
    int lo = -1, hi = n - 1, mid = 0;
    while (hi - lo > 1) {
        mid = lo + (hi - lo)/2;
        if (v[mid] <= x) {
            lo = mid;
        }
        else
            hi = mid;
    }
    return v[lo] == x ? lo : -1 ;
}

快排

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int partition(vector<int> &v, int lo, int hi){
    if(lo>=hi) return lo;
    int pi = lo;
    while (lo<hi) {
        //hi先往lo方向走，双向走必到同一点
        while (lo<hi && v[hi] >= v[pi]) {
            hi--;
        }
        while (lo<hi && v[lo] <= v[pi]) {
            lo++;
        }
        swap(v[lo], v[hi]);//交换过后仍循环符合条件
    }
    swap(v[pi], v[lo]);
    return lo;
}
void qS(vector<int> &v, int lo, int hi){
    if (lo>=hi) {
        return;
    }
    int mid = partition(v, lo, hi);
    qS(v, lo, mid - 1);
    qS(v, mid + 1, hi);
    return;
}
void qS2(vector<int> &v, int lo, int hi){
    if (lo >= hi) {
        return;
    }
    
    //partition
    int i = lo, j = hi, randompi = lo + rand()%(hi -lo);
    //1 pi一定还是最左，只是rand交换一下最左元素
    swap(v[randompi], v[i]);
    
    int pi = i;
    //2 写入一个函数增加变量不然递归时lo和hi的值会变
    while (i < j) {
        //3 一定先从对立面
        while (i < j && v[j] >= v[pi]) {
            j--;
        }
        while (i < j && v[i] <= v[pi]) {
            i++;
        }
        swap(v[i], v[j]);
    }
    swap(v[i], v[pi]);
    //4 返回partition
    int mid = i;
    
    qS2(v, lo, mid - 1);
    qS2(v, mid+1, hi);
    return;
}

归并

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void mergeV(vector<int> &v, int lo, int mid, int hi){
    if (lo >= hi) {
        return;
    }
    vector<int> tmp(v);
    //xxxidx要相对从0
    int cur1 = lo, cur2 = mid+1, idx = lo;
    while (cur1 <= mid && cur2 <= hi) {
        if (tmp[cur1] <= tmp[cur2]) {
            v[idx++] = tmp[cur1++];
        }
        else
            v[idx++] = tmp[cur2++];
    }
    while (cur1 <= mid) {
        v[idx++] = tmp[cur1++];
    }
    while (cur2 <= hi) {
        v[idx++] = tmp[cur2++];
    }
    return;
}

void mergeS(vector<int> &v, int lo, int hi){
    if (lo >= hi) {
        return;
    }
    //partition
    int mid = lo + (hi - lo)/2;
    //xxx和qS不同mid不是中立不传入mid-1
    mergeS(v, lo, mid);
    mergeS(v, mid + 1, hi);
    mergeV(v, lo, mid, hi);

}

DFS

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void cdfs(vector<int> &cur, set<vector<int>> &res, vector<int> &visit, vector<int> &v, int dep, int len, int sum, int vdx){
    if (dep == 4) {
        if (sum == 0) {
            res.insert(cur);
            return;
        }
        return;
    }
    //扩展方式
    for (; vdx < len; ++vdx) {
        if (visit[vdx] == 0) {
            visit[vdx] = 1;
            cur.push_back(v[vdx]);
            cdfs(cur, res, visit, v, dep + 1, len, sum + v[vdx], vdx + 1);
            visit[vdx] = 0;
            cur.pop_back();
        }
    }
    return;
    
}

vector<vector<int> > threeSum(vector<int> &num) {
    vector<vector<int>> res;
    vector<int> cur;
    set<vector<int>> tmp;
    vector<int> visit(num.size(), 0);
    if (num.size() < 1) {
        return res;
    }
    sort(num.begin(), num.end());
    cdfs(cur, tmp, visit, num, 1, num.size(), 0, 0);
    for(auto i : tmp)
        res.push_back(i);
    return res;
    
}