First Position of Target

class Solution {
    /**
     * @param nums: The integer array.
     * @param target: Target to find.
     * @return: The first position of target. Position starts from 0.
     */
    public int binarySearch(int[] nums, int target) {
        //write your code here
        if(nums.length == 0)
            return -1;
            
        int s = 0;
        int e = nums.length-1;
        int m;
        
        while(s+1 < e){
            m = s + (e - s)/2;
            
            if(nums[m] == target)
                e = m;//有duplicate 所以不能直接返回，此处移动e 好保留前面第一个出现的位置
            else if(nums[m] < target) 
                s = m;
            else
                e = m;
           
        }
        
        if(nums[s] == target)
            return s;
            
        if(nums[e] == target)
            return e;
            
            return -1;
    }
}