7_sortAStack.md

Sort a stack

Recursive Solution

1. Insert function.

   • If stack is empty OR x is the largest element, then we can directly insert it at last position.
   • else we need to insert x at it's correct position.
   • Get the top element of the stack and reduce its size by 1.
   • Insert x recursively.
   • finally insert value which is poped.

2. Sort function.

   • If stack contains only one element then it is sorted iteself.Return.
   • Get the last element of the stack.
   • Reduce the size of input for recursion.
   • Recursively sort remaining the stack which has one element less.
   • Insert the element in sorted stack at its right position.

Code

#include <bits/stdc++.h>

void insertSorted(stack<int>& st, int x) {
    if (st.empty() || st.top() <= x) {
        st.push(x);
    } else {
        int temp = st.top();
        st.pop();
        insertSorted(st, x);
        st.push(temp);
    }
}

void sortStack(stack<int>& stack)
{
    if (stack.empty())
        return;
    int x = stack.top();
    stack.pop();
    sortStack(stack);
    insertSorted(stack, x);
}