学习笔记---算法

今天无聊又开始在看算法了，可惜看那么久还是在看简单的排序，把今天的成果贴出来吧

package sort;

//冒泡排序
public class BubbleSort {
    //the bubblesort method
    public static int[] bubbleSort(int[] list) {
        //the boolean varaible for check is the last varaible of the list
        //if true,the sort method will be stoped.
        boolean isNeedNextSort = true;
        //if index equals list's length
        for (int index = 1;index < list.length && isNeedNextSort;index++) {
            //let the boolean varaible be false;
            isNeedNextSort = false;
            //if  the varaible nor lower than list.length-index,
            //in other word,the list's length is equals index,
            //the next sort can be stop.
            for(int varaible = 0;varaible < list.length-index;varaible++){
                if(list[varaible] > list[varaible+1]){
                    int temp = list[varaible];
                    list[varaible] = list[varaible+1];
                    list[varaible+1] = temp;
                }
                //there will be a next sort
                isNeedNextSort = true;
            }
        }
        return list;
    }
}

package sort;

//归并排序,至今还有点迷糊，第二次mergeSort()怎么执行的
public class MergeSort {
    
    /**
     * The MegerSort Method 
     */
    
    public static int[] mergeSort(int[] list){
        /**
         * MergeSort the first half
         */
        
        //if there has more than 1 elements
        if(list.length > 1){
            //create a array which can contain the list's half
            int[] firstHalf = new int[list.length/2];
            //arraycopy:copy the first half of list to firstHalf
            System.arraycopy(list, 0, firstHalf, 0, list.length/2);
            //mergesort the half too,until it's elements is 1
            mergeSort(firstHalf);
            
            /**
             * MergeSort the secondhalf
             */
            
            //figureout the secondHalf array's length
            int secondHalfLength = list.length-list.length/2;
            //create the secondhalf array
            int[] secondHalf = new int[secondHalfLength];
            //arraycopy:copy the second half of list to secondHalf
            System.arraycopy(list, list.length/2, secondHalf, 0, secondHalfLength);
            //mergesort the half too,until it's elements is 1
            mergeSort(secondHalf);
            
            int[] temp = merge(firstHalf,secondHalf);
            System.arraycopy(temp, 0, list, 0, temp.length);
        }
        return list;
    }
    
    /**
     * The Meger Method
     */
    
    private static int[] merge(int[] firstHalf, int[] secondHalf) {
        //create a array called temp,it's length equals list's
        int[] temp = new int[firstHalf.length+secondHalf.length];
        //create three index of three array
        int firstHalf_index = 0;
        int secondHalf_index = 0;
        int temp_index = 0;
        //all the rest code is just for sort the all elements
        while (firstHalf_index < firstHalf.length && secondHalf_index < secondHalf.length) {
            if (firstHalf[firstHalf_index] <= secondHalf[secondHalf_index]) {
                temp[temp_index] = firstHalf[firstHalf_index];
                firstHalf_index++;
            } else {
                temp[temp_index] = secondHalf[secondHalf_index];
                secondHalf_index++;
            }
            temp_index++;
        }
 
        if (firstHalf_index == firstHalf.length) {
            while (secondHalf_index < secondHalf.length) {
                temp[temp_index++] = secondHalf[secondHalf_index++];
            }
        } else { 
            // secondHalf_index==secondHalf.length
            while (firstHalf_index < firstHalf.length) {
                temp[temp_index++] = firstHalf[firstHalf_index++];
            }
        }
        return temp;
    }
}