这是蓝桥杯经典题“拉马车”，直接模拟即可

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

static char qA[10005], qB[10005];

int main() {
char buf[64];
scanf("%s", buf);
int hA = 0, tA = 0;
for (int i = 0; buf[i]; i++) qA[tA++] = buf[i];
scanf("%s", buf);
int hB = 0, tB = 0;
for (int i = 0; buf[i]; i++) qB[tB++] = buf[i];

static char table[10005];
int top = 0;
int cnt[128] = {0};
int turn = 0;
int steps = 0;

while (hA < tA && hB < tB) {
    if (++steps > 10000) { puts("-1"); return 0; }
    
    char c;
    if (turn == 0) c = qA[hA++];
    else c = qB[hB++];
    
    if (cnt[(int)c] > 0) {
        if (turn == 0) {
            qA[tA++] = c;
            while (true) {
                char t = table[--top];
                cnt[(int)t]--;
                qA[tA++] = t;
                if (t == c) break;
            }
        } else {
            qB[tB++] = c;
            while (true) {
                char t = table[--top];
                cnt[(int)t]--;
                qB[tB++] = t;
                if (t == c) break;
            }
        }
    } else {
        table[top++] = c;
        cnt[(int)c]++;
        turn ^= 1;
    }
}

if (hA >= tA) {
    for (int i = hB; i < tB; i++) putchar(qB[i]);
} else {
    for (int i = hA; i < tA; i++) putchar(qA[i]);
}
putchar('\n');
return 0;

}