剑指offer第五天

1.包含min函数的栈

一个比较简单的模拟栈的操作

cpp 复制代码

class Solution {
public:
    void push(int value) {
        st[op++] = value;
    }
    void pop() {
        if(op)op--;
    }
    int top() {
        return st[op-1];
    }
    int min() {
        int mi = 10001;
        for(int i = 0;i<op;++i)mi = std::min(mi,st[i]);
        return mi;
    }
private:
    int st[310]{};
    int op = 0;
};

2.栈的压入、弹出序列

思考一下这道题的做法，无非就是用栈来模拟一下这个过程，比较简单

借助一个辅助栈，来完成
1. 按照pushV的顺序先插入到辅助栈中，和popV的元素去匹配

cpp 复制代码

class Solution {
public:
    bool IsPopOrder(vector<int>& pushV, vector<int>& popV) {
        // write code here
        stack<int>st;
        int n = pushV.size(),idx = 0;
        for(int i = 0;i<n;++i)
        {
            st.push(pushV[i]);
            if(st.top() == popV[idx])
            {
                while(!st.empty()&&st.top() == popV[idx])
                {
                    st.pop(),idx++;
                }
            }
        }
        return st.empty();
    }
};

3.翻转单词序列

采用两步反转法，先把整体的字符串反转，再把每个单词反转

cpp 复制代码

class Solution {
    void reverse(string& s, int l, int r)
    {
        while (l < r)
        {
            swap(s[l++], s[r--]);
        }
    }
public:
    string ReverseSentence(string str) {
        int n = str.size();
        //时间复杂度O(n)，空间复杂度O(1)
        //两步反转法
        reverse(str, 0, n - 1);
        int l = 0, r = 0;
        while (l < n)
        {
            while (l<n&&str[l] == ' ')l++;
            if (l >= n)break;
            r = l;
            while (r<n&&str[r] != ' ')r++;
            reverse(str, l, r - 1);
            l = r;
        }
        return str;
    }
};

4.滑动窗口的最大值

采用的是 set + 仿函数 的方式来解决这个问题，先把前size个数字加入到set中去，然后继续向后遍历，取出最大的，插入一个元素再删除一个元素

cpp 复制代码

struct compare {
    bool operator()(const std::pair<int, int>& a,
                    const std::pair<int, int>& b) const {
        if (a.first != b.first)
            return a.first > b.first;
        else
            return a.second < b.second;
    }
};
class Solution {
  public:
    vector<int> maxInWindows(vector<int>& num, int size) {
        // write code here
        set<pair<int, int>, compare>q; //从大到小进行排序
        vector<int> result;
        int n = num.size();
        if (size == 0 || size>n)return result;
        int i = 0;
        for (; i < size; ++i) {
            q.insert({num[i], i});
        }

        for (; i < n; ++i) {
            result.push_back(q.begin()->first);
            q.erase({num[i - size], i - size});
            q.insert({num[i], i});
        }
        result.push_back(q.begin()->first);
        return result;
    }
};

5.数组中出现次数超过一半的数字

方法一：双指针

cpp 复制代码

class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param numbers int整型vector 
     * @return int整型
     */
    int MoreThanHalfNum_Solution(vector<int>& numbers) {
        // write code here
        sort(numbers.begin(),numbers.end());
        int n = numbers.size();
        int half = n/2;
        //双指针
        int left = 0;
        while(left<n)
        {
            int val = numbers[left];
            int right = left;
            while(right<n&&numbers[right]==val)right++;
            if(right-left>half)return val;
            left = right;
        }
        return -1;
    }
};

方法二：分析

cpp 复制代码

class Solution {
public:
    int MoreThanHalfNum_Solution(vector<int>& numbers) {
        // write code here
        sort(numbers.begin(),numbers.end());
        return numbers[numbers.size()/2];
    }
};

6.旋转数组的最小数字

python 复制代码

class Solution:
    def minNumberInRotateArray(self , nums: List[int]) -> int:
        # write code here
        return min(nums)

7.字符串的排列

cpp 复制代码

class Solution {
    vector<string>result;
    int vis[15]{};
    string path;
    int n;
    void dfs(string&s,int u)
    {
        if(u == n)
        {
            result.push_back(path);
            return;
        }
        for(int i = 0;i<n;++i)
        {
            if(i>0&&s[i]==s[i-1]&&!vis[i-1])continue;
            if(!vis[i])
            {
                vis[i] = true;
                path.push_back(s[i]);
                dfs(s,u+1);
                path.pop_back();
                vis[i] = false;
            }
        }
    }
public:
    vector<string> Permutation(string str) {
        // write code here
        n = str.size();
        sort(str.begin(),str.end());
        dfs(str,0);
        return result;
    }
};

8.数字序列的某一位的数字

python 复制代码

def find_nth_digit(n):
    length = 1
    count = 9
    start = 1
    while n > length * count:
        n -= length * count
        length += 1
        count *= 10
        start *= 10
    num = start + (n - 1) // length
    digit_index = (n - 1) % length
    return int(str(num)[digit_index])
class Solution:
    def findNthDigit(self , n: int) -> int:
        # write code here
        return find_nth_digit(n)