#include <iostream>
#include <map>
#include <algorithm>
#include <limits.h>
#include <assert.h>
#include <stack>
using namespace std;
int selectK(int num[], int k, int l, int r) {assert(k <= (r - l + 1) && k >= 1);int mid = (l + r) / 2, i = l, j = r;while (i <= j) {while (num[i] < num[mid]) {++i;}while (num[j] > num[mid]) {--j;}if (i <= j) {swap(num[i],num[j]);++i,--j;}}if (k == i - l)return num[i - 1];else if (k < i - l)return selectK(num, k, l, i -1);else if (k == j + 1 - l)return num[j + 1];elsereturn selectK(num, k - (j - l + 2), j + 2, r);
}
void pathcompress(char* str) {int i = 0, j = 0;char prev = '\0';stack<int> offset;offset.push(0);while(str[i]) {if (prev == '/') {if (str[i] == '.' && (str[i + 1] == '/' || str[i + 1] == '\0')) {prev = str[i + 1], i += 1;}else if (str[i] == '.' && str[i + 1] == '.' && (str[i + 2] == '/' || str[i + 2] == '\0')) {i += 2;if (offset.empty()) {cout << "error" << endl;return;            }j = offset.top();offset.pop();if (offset.empty() && str[0] == '/') {cout << "error" << endl;return;}}else if (str[i] == '/') {prev = str[i++];}else {offset.push(j);prev = str[i];str[j++] = str[i++];        }}else {prev = str[i];str[j++] = str[i++];}}if (j >=3 && str[j - 1] == '/' && str[j-2] == '/')str[j-2] = '\0';else if (j >= 2 && str[j - 1] == '/')str[j-1] = '\0';elsestr[j] = '\0';if (str[0] == '\0'){str[0] = '.';str[1] = '\0';}}int main()
{int num[] = {3,2,1,4,5};int res1 = selectK(num, 1, 0, 4);int res2 = selectK(num, 2, 0, 4);int res3 = selectK(num, 3, 0, 4);int res4 = selectK(num, 4, 0, 4);int res5 = selectK(num, 5, 0, 4);// int res6 = selectK(num, 6, 0, 4);//char str[] = "";char str[] = "/.abc/xxx./abc/bacd/.././bcd/fsgs/../../../x/y/z/../../../..";pathcompress(str);printf("%s\n",str);return 0;
}

BUT this isn't right, For example: char str[] = "../../../etc/xyz/../abc"; You couldn't print error here. The right solution is:

#include <iostream>
#include <map>
#include <algorithm>
#include <limits.h>
#include <assert.h>
#include <stack>
using namespace std;
int selectK(int num[], int k, int l, int r) {assert(k <= (r - l + 1) && k >= 1);int mid = (l + r) / 2, i = l, j = r;while (i <= j) {while (num[i] < num[mid]) {++i;}while (num[j] > num[mid]) {--j;}if (i <= j) {swap(num[i],num[j]);++i,--j;}}if (k == i - l)return num[i - 1];else if (k < i - l)return selectK(num, k, l, i -1);else if (k == j + 1 - l)return num[j + 1];elsereturn selectK(num, k - (j - l + 2), j + 2, r);
}void pathcompress2(char* str) {stack<int> path;int i = 0, j = 0;bool isRoot = (str[0] == '/');char prev = '\0';int len = strlen(str);if (!(len >= 3 && str[0] == '.' && str[1] == '.' && str[2] == '/'))path.push(0);while(str[i]) {if (prev == '/') {if (str[i] == '.' && (str[i+1] == '/' || str[i+1] == '\0')) {prev = '/';if (str[i+1] == '\0') {str[j] = '\0';break;}i+=2;}else if (str[i] == '.' && str[i+1] == '.' && (str[i+2] == '/' || str[i+2] == '\0')) {if (path.empty()) {str[j++] = str[i];str[j++] = str[i+1];str[j++] = str[i+2];        prev = '/';if (str[i+2] == '\0') {str[j] = '\0';break;}i+=3;}else {j = path.top();path.pop();if (path.empty() && isRoot) {  //  The case : cd /..printf("Error\n");return;}if (str[i+2] == '\0') {str[j] = '\0';break;}prev = '/';i += 3;}}else if (str[i] == '/')prev = str[i++];else {prev = str[i];path.push(j);str[j++] = str[i++];} }else {prev = str[i];str[j++] = str[i++];}}if (j >= 2 && str[j - 1] == '/')str[j-1] = '\0';elsestr[j] = '\0';if (str[0] == '\0'){str[0] = '.';str[1] = '\0';}}int main()
{int num[] = {3,2,1,4,5};int res1 = selectK(num, 1, 0, 4);int res2 = selectK(num, 2, 0, 4);int res3 = selectK(num, 3, 0, 4);int res4 = selectK(num, 4, 0, 4);int res5 = selectK(num, 5, 0, 4);// int res6 = selectK(num, 6, 0, 4);char str[] = "/.abc/xxx./abc/bacd/.././bcd/fsgs/../../../x/y/z/../../../../.././././xda";//char str[] = "asdf/.abc/xxx./abc/bacd/.././bcd/fsgs/../../../x/y/z/../../../../../../.././../.././././";//char str[] = "/xyz/./bcd/fsgs/../../../x/y/z/../../../..";//char str[] = "../../../etc/xyz/../abc/.asdf/../../../../";pathcompress2(str);printf("%s\n",str);return 0;
}

The concise version is :

void pathcompress2(char* str) {stack<int> path;int i = 0, j = 0, len = strlen(str);bool isRoot = (str[0] == '/');char prev = '\0';if (!(len >= 3 && str[0] == '.' && str[1] == '.' && (str[2] == '/' || str[2] == '\0'))path.push(0);while(str[i]) {if (prev == '/') {if (str[i] == '.' && (str[i+1] == '/' || str[i+1] == '\0')) {prev = '/';if (str[i+1] == '\0') {str[j] = '\0';break;}i+=2;}else if (str[i] == '.' && str[i+1] == '.' && (str[i+2] == '/' || str[i+2] == '\0')) {if (path.empty()) {str[j++] = str[i],str[j++] = str[i+1],str[j++] = str[i+2],prev = '/';}else {j = path.top();path.pop();if (path.empty() && isRoot) {  //  The case : cd /..printf("Error\n");return;}}if (str[i+2] == '\0') {str[j] = '\0';break;}prev = '/',i += 3;}else if (str[i] == '/')prev = str[i++];else {prev = str[i],path.push(j),str[j++] = str[i++];} }else {prev = str[i],str[j++] = str[i++];}}  if (j >= 2 && str[j - 1] == '/')str[j-1] = '\0';elsestr[j] = '\0';if (str[0] == '\0'){str[0] = '.',str[1] = '\0';}
}