670. Maximum Swap

package LeetCode_670

/**
 * 670. Maximum Swap
 * https://leetcode.com/problems/maximum-swap/description/
 *
 * Given a non-negative integer, you could swap two digits at most once to get the maximum valued number.
 * Return the maximum valued number you could get.

Example 1:
Input: 2736
Output: 7236
Explanation: Swap the number 2 and the number 7.
 * */
class Solution {
    fun maximumSwap(num: Int): Int {
        val numString = num.toString()
        var leftIndex = -1
        var rightIndex = -1
        var maxNum = -1
        var maxIndex = -1
        for (i in numString.length - 1 downTo 0) {
            if (maxNum < numString[i].toInt()) {
                maxNum = numString[i].toInt()
                maxIndex = i
            }
            if (maxNum > numString[i].toInt()) {
                leftIndex = i
                rightIndex = maxIndex
            }
        }
        if (leftIndex == -1) {//for example: 9973
            return num
        }
        val chars = numString.toCharArray()
        swap(chars, leftIndex, rightIndex)
        return arrayToNum(chars)
    }

    private fun arrayToNum(array: CharArray): Int {
        var result = 0
        for (item in array) {
            if (item.equals("")) {
                continue
            }
            result *= 10
            result += (item.toInt() - '0'.toInt())
        }
        return result
    }

    private fun swap(str: CharArray, start: Int, end: Int) {
        val temp = str[end]
        str[end] = str[start]
        str[start] = temp
    }
}