Binary Search

The original case/problem could be found on leedcode

case 744 (classical case)

var nextGreatestLetter = function(letters, target) {
    let low=0
    let high=letters.length;
    while(low<=high){
        let mid=Math.floor(low+(high-low)/2) // import math form, the code of binary search!!!!
        if(letters[mid]>target){
            high=mid-1
        }
        else{
            low=mid+1
        }
    }
    return low>=letters.length?letters[0]:letters[low]
};

class Solution {

    /**
     * @param String[] $letters
     * @param String $target
     * @return String
     */
    function nextGreatestLetter($letters, $target) {
        $left = 0;
        $right = count($letters);

    while ($left<=$right) {
        $mid = floor($left+($right-$left)/2);
        if ($letters[$mid]>$target) $right = $mid-1;
        else $left = $mid+1;
    }

    if ($left>=count($letters)) return $letters[0];
    else return $letters[$left];
    }
}