Leetcode 163. Missing Ranges

[locked]

# Time:  O(n)
# Space: O(1)
#
# Given a sorted integer array where the range of elements are [lower, upper] inclusive,
# return its missing ranges.
# 
# For example, given [0, 1, 3, 50, 75], lower = 0 and upper = 99, 
# return ["2", "4->49", "51->74", "76->99"].
#

class Solution:
    # @param A, a list of integers
    # @param lower, an integer
    # @param upper, an integer
    # @return a list of strings
    def findMissingRanges(self, A, lower, upper):
        ranges = []
        pre = lower - 1
        
        for i in xrange(len(A) + 1):
            if i == len(A):
                cur = upper + 1
            else:
                cur = A[i]
            
            if cur - pre >= 2:
                ranges.append(self.getRange(pre + 1, cur - 1))
                
            pre = cur
            
        return ranges
    
    def getRange(self, lower, upper):
        if lower == upper:
            return "{}".format(lower)
        else:
            return "{}->{}".format(lower, upper)