1375.bulb-switcher-iii.kt

/*
 * @lc app=leetcode id=1375 lang=kotlin
 *
 * [1375] Bulb Switcher III
 *
 * https://leetcode.com/problems/bulb-switcher-iii/description/
 *
 * algorithms
 * Medium (55.96%)
 * Likes:    34
 * Dislikes: 6
 * Total Accepted:    5.4K
 * Total Submissions: 9.7K
 * Testcase Example:  '[2,1,3,5,4]'
 *
 * There is a room with n bulbs, numbered from 1 to n, arranged in a row from
 * left to right. Initially, all the bulbs are turned off.
 * 
 * At moment k (for k from 0 to n - 1), we turn on the light[k] bulb. A bulb
 * change color to blue only if it is on and all the previous bulbs (to the
 * left) are turned on too.
 * 
 * Return the number of moments in which all turned on bulbs are blue.
 * 
 * 
 * Example 1:
 * 
 * 
 * 
 * 
 * Input: light = [2,1,3,5,4]
 * Output: 3
 * Explanation: All bulbs turned on, are blue at the moment 1, 2 and 4.
 * 
 * 
 * Example 2:
 * 
 * 
 * Input: light = [3,2,4,1,5]
 * Output: 2
 * Explanation: All bulbs turned on, are blue at the moment 3, and 4
 * (index-0).
 * 
 * 
 * Example 3:
 * 
 * 
 * Input: light = [4,1,2,3]
 * Output: 1
 * Explanation: All bulbs turned on, are blue at the moment 3 (index-0).
 * Bulb 4th changes to blue at the moment 3.
 * 
 * 
 * Example 4:
 * 
 * 
 * Input: light = [2,1,4,3,6,5]
 * Output: 3
 * 
 * 
 * Example 5:
 * 
 * 
 * Input: light = [1,2,3,4,5,6]
 * Output: 6
 * 
 * 
 * 
 * Constraints:
 * 
 * 
 * n == light.length
 * 1 <= n <= 5 * 10^4
 * light is a permutation of  [1, 2, ..., n]
 * 
 * 
 */

// @lc code=start
class Solution {
    fun numTimesAllBlue(light: IntArray): Int {
        var sum = 0
        var ans = 0
        var mm = IntArray(light.size+1)
        for (i in 1 .. light.size) {
            sum += i
            mm[i] = sum
        }
        sum = 0
        for (i in 0 until light.size) {
            sum += light[i]
            if (sum == mm[i+1]) ans++
        }
        return ans
    }
}
// @lc code=end