Underscorify Substring

def underscorifySubstring(string, substring):
    locations = collapse(getLocations(string, substring))
    return underscorify(string, locations)

def getLocations(string, substring):
    locations = []
    startIdx = 0;
    while startIdx < len(string):
        nextIdx = string.find(substring, startIdx);
        if nextIdx != -1: #if substring is found, add the start index and the end index
            locations.append([nextIdx, nextIdx + len(substring)])
            startIdx = nextIdx + 1 #continue to traverse the next one
        else:
            break # if not found, break
    return locations

def collapse(locations): # address the special cases: 1. overlap 2. sit side by side
    if not len(locations): # if the locations array is empty, return []
        return locations
    newLocations = [locations[0]] # newLocations starts from the first element in the 2d array
    previous = newLocations[0] #initiate the previous(the first position)
    for i in range(1, len(locations)): # check the following positions
        current = locations[i] #current defines the following position, from the second array in locations
        if current[0] <= previous[1]: #[1,3] [2,4] current[0] = 2, previous[1] = 3, overlapping
            previous[1] = current[1] #[1,3]->[1,4] # update previous
        else: # no overlapping or sit side by side
            newLocations.append(current) # append the whole currect array
            previous = current # update the previous to current
    return newLocations

def underscorify(string, locations): # add underscore
    locationsIdx = 0
    stringIdx = 0
    inBetweenUnderscores = False
    finalChars = []
    i = 0
    while stringIdx < len(string) and locationsIdx < len(locations): # while we are in the middle od string and the locations array
        if stringIdx == locations[locationsIdx][i]:
            finalChars.append("_")
            inBetweenUnderscores = not inBetweenUnderscores # finished adding _ for one array
            if not inBetweenUnderscores: #false(initial)-> true -> false(one pass finished, move to another) -> true->false
                locationsIdx += 1 # go to the next array
            i = 0 if i == 1 else 1 # if i =1, ->i = 0;   if i = 0, ->i = 1, i have two potions: 0 or 1
        finalChars.append(string[stringIdx]) # if stringIdx != locations[locationsIdx][i], append the substring directly
        stringIdx += 1
    if locationsIdx < len(locations): # finished traversing the string 
        finalChars.append("_")
    elif stringIdx < len(string): # finished traversing the locations
        finalChars.append(string[stringIdx:])
    return "".join(finalChars)