基于快速排序思想partition查找第K大的数或者第K小的数。

function quickSort(a,left,right){if(left&＃61;&＃61;right)return;let key&＃61;partition(a,left,right);//选出key下标if(left<key){quickSort(a,left,key-1);//对key的左半部分排序
}if(key<right){quickSort(a,key&＃43;1,right)//对key的右半部份排序
}
}
function partition(a,left,right){let key&＃61;a[left];//一开始让key为第一个数while(left//扫描一遍while(key<&＃61;a[right]&&left//如果key小于a[right]&＃xff0c;则right递减&＃xff0c;继续比较right--;}[a[left],a[right]]&＃61;[a[right],a[left]];//交换while(key>&＃61;a[left]&&left//如果key大于a[left]&＃xff0c;则left递增&＃xff0c;继续比较left&＃43;&＃43;;}[a[left],a[right]]&＃61;[a[right],a[left]];//交换
}return left;//把key现在所在的下标返回
}

function findKmax(a,k){let left&＃61;0,right&＃61;a.length-1;let key&＃61;partition(a,left,right);let len&＃61;a.length-key;while(len!&＃61;k){if(len>k){key&＃61;partition(a,key&＃43;1,right);}else{key&＃61;partition(a,left,key-1);}len&＃61;a.length-key;}return a[key];
}
function partition(a,left,right){let key&＃61;a[left];//一开始让key为第一个数while(left//扫描一遍while(key<&＃61;a[right]&&left//如果key小于a[right]&＃xff0c;则right递减&＃xff0c;继续比较right--;}[a[left],a[right]]&＃61;[a[right],a[left]];//交换while(key>&＃61;a[left]&&left//如果key大于a[left]&＃xff0c;则left递增&＃xff0c;继续比较left&＃43;&＃43;;}[a[left],a[right]]&＃61;[a[right],a[left]];//交换
}return left;//把key现在所在的下标返回
}

function findKmin(a,k){//查找第K小的数let left&＃61;0,right&＃61;a.length-1;let key&＃61;partition(a,left,right);while(key!&＃61;k-1){if(key>k-1){key&＃61;partition(a,left,key-1);}else{key&＃61;partition(a,key&＃43;1,right);}}return a[key];
}
function partition(a,left,right){let key&＃61;a[left];//一开始让key为第一个数while(left//扫描一遍while(key<&＃61;a[right]&&left//如果key小于a[right]&＃xff0c;则right递减&＃xff0c;继续比较right--;}[a[left],a[right]]&＃61;[a[right],a[left]];//交换while(key>&＃61;a[left]&&left//如果key大于a[left]&＃xff0c;则left递增&＃xff0c;继续比较left&＃43;&＃43;;}[a[left],a[right]]&＃61;[a[right],a[left]];//交换
}return left;//把key现在所在的下标返回
}