双指针算法核心概念与 C++ 经典例题解析 | 极客日志

C++算法

双指针算法核心概念与 C++ 经典例题解析

双指针算法通过两个指针配合变化高效解决问题。内容涵盖合并有序数组、完美数列、反转元音字母、验证回文串变体、最接近三数之和及三数之和多种可能等六个经典 C++ 例题。包含算法概念解析与完整代码实现，旨在帮助读者掌握双指针在数组处理与字符串操作中的具体应用技巧。

念念不忘发布于 2026/3/16更新于 2026/4/2611 浏览

双指针算法核心概念与 C++ 经典例题解析

概念

双指针，顾名思义，就是用两个指针解决问题。

有些问题用单指针会出现超时等问题，这时就需要用到双指针。

双指针由两个指针组成，一般是左右指针，或前后指针。

通过两个指针配合变化，用更短的时间高效解决问题。

题目

合并有序数组

#include <bits/stdc++.h>
#define ll long long
using namespace std;

int la, lb, lab;
int a[20010], b[10010];

int main() {
    cin >> la;
    for (int i = 1; i <= la; i++) {
        cin >> a[i];
    }
    cin >> lb;
    for (int i = 1; i <= lb; i++) {
        cin >> b[i];
    }
    lab = la + lb;
    int ai = la + 1, bi = lb + 1, abi = lab + 1;
    while (ai > 1 && bi > 1) {
        if (a[ai - 1] > b[bi - 1]) a[--abi] = a[--ai];
        else a[--abi] = b[--bi];
    }
    while (bi > 1) a[--abi] = b[--bi];
    for (int i = 1; i <= lab; i++) cout << a[i] << " ";
    return 0;
}

完美数列

#include <bits/stdc++.h>

  std;

 n, p;
 a[];

{
    cin >> n >> p;
     ( i = ; i <= n; i++) {
        cin >> a[i];
    }
    (a + , a + n + );
     fast = , slow = ;
     ma, mi;
    mi = a[slow];
     mama = ;
     (fast < n) {
         (fast < n) {
            ma = a[++fast];
             x = mi * p;
             (ma <= x) mama = (mama, fast - slow + );
             ;
        }
         (slow < fast) {
            mi = a[++slow];
             x = mi * p;
             (ma <= x) {
                mama = (mama, fast - slow + );
                ;
            }
        }
    }
    cout << mama;
     ;
}

#include <bits/stdc++.h>
#define ll long long
using namespace std;

string a;
char yy[] = {'a', 'e', 'i', 'o', 'u'};

int main() {
    cin >> a;
    int n = a.size();
    int l = 0, r = n - 1;
    while (l < r) {
        bool lf = false;
        bool rf = false;
        for (int i = 0; i < 5; i++) {
            if (a[l] == yy[i]) lf = true;
            if (a[r] == yy[i]) rf = true;
        }
        if (lf == false) l++;
        if (rf == false) r--;
        if (lf == true && rf == true) {
            swap(a[l], a[r]);
            l++;
            r--;
        }
    }
    cout << a;
    return 0;
}

#include <bits/stdc++.h>
#define ll long long
using namespace std;

string a;
int n;
bool chushi();
bool hou(int, int);

int main() {
    cin >> a;
    n = a.size();
    if (chushi()) cout << "true";
    else cout << "false";
    return 0;
}

bool chushi() {
    int l = 0, r = n - 1;
    while (l < r) {
        if (a[l] == a[r]) l++, r--;
        else {
            if ((hou(l + 1, r) || hou(l, r - 1))) return true;
            else return false;
        }
    }
    return true;
}

bool hou(int l, int r) {
    while (l < r) {
        if (a[l] == a[r]) l++, r--;
        else return false;
    }
    return true;
}

#include <bits/stdc++.h>
#define ll long long
using namespace std;

int n, a[10010], target, misum, micha;

int main() {
    micha = 999999999;
    cin >> n;
    for (int i = 1; i <= n; i++) {
        cin >> a[i];
    }
    cin >> target;
    sort(a + 1, a + n + 1);
    for (int i = 1; i <= n - 2; i++) {
        int l = i + 1, r = n;
        while (l < r) {
            int sum = a[l] + a[i] + a[r];
            int cha = sum - target;
            if (cha < 0) cha = target - sum, l++;
            else r--;
            if (cha < micha) {
                micha = cha;
                misum = sum;
            }
        }
    }
    cout << misum;
    return 0;
}

#include <bits/stdc++.h>
#define ll long long
using namespace std;

int n, a[10010], target;
int cnt;

int main() {
    cin >> n;
    for (int i = 1; i <= n; i++) {
        cin >> a[i];
    }
    cin >> target;
    sort(a + 1, a + n + 1);
    for (int i = 1; i <= n - 2; i++) {
        int l = i + 1;
        int r = n;
        while (l < r) {
            int sum = a[i] + a[l] + a[r];
            if (sum == target) {
                cnt++;
                int p = r - 1;
                while (a[p] == a[r] && p > l) {
                    cnt++;
                    p--;
                }
                l++;
            }
            if (sum > target) r--;
            if (sum < target) l++;
        }
    }
    cout << cnt;
    return 0;
}