http://stackoverflow.com/questions/28223580/how-to-get-the-least-number-after-deleting-k-digits-from-the-input-number#28223817

solve.py

def solve(number, k):
    n = len(number)
    stack = []
    for i, digit in enumerate(str(number)):
        while stack and digit < stack[-1] and n-i+len(stack) > k:
            stack.pop()
        if len(stack) < n - k:
            stack.append(digit)
    return int(''.join(stack[:k]))
        
print solve('24635', 2)
print solve('123456', 2)
print solve('654321', 2)
print solve('122212221', 2)