# Median of medians无序数组寻找中位数最差O(n)复杂度JS实现

``````export const selectMedian = (arr, compare) => {
return selectK(arr, Math.floor(arr.length / 2), compare);
};

export const selectK = (arr, k, compare) => {
if (!Array.isArray(arr) || arr.length === 0 || arr.length - 1 < k) {
return;
}
if (arr.length === 1) {
return arr[0];
}
let idx = selectIdx(arr, 0, arr.length - 1, k, compare || defaultCompare);
return arr[idx];
};

const partition = (arr, left, right, pivot, compare) => {
let temp = arr[pivot];
arr[pivot] = arr[right];
arr[right] = temp;
let track = left;
for (let i = left; i < right; i++) {
// if (arr[i] < arr[right]) {
if (compare(arr[i], arr[right]) === -1) {
let t = arr[i];
arr[i] = arr[track];
arr[track] = t;
track++;
}
}
temp = arr[track];
arr[track] = arr[right];
arr[right] = temp;
return track;
};

const selectIdx = (arr, left, right, k, compare) => {
if (left === right) {
return left;
}
let dest = left + k;
while (true) {
let pivotIndex =
right - left + 1 <= 5
? Math.floor(Math.random() * (right - left + 1)) + left
: medianOfMedians(arr, left, right, compare);
pivotIndex = partition(arr, left, right, pivotIndex, compare);
if (pivotIndex === dest) {
return pivotIndex;
} else if (pivotIndex < dest) {
left = pivotIndex + 1;
} else {
right = pivotIndex - 1;
}
}
};

const medianOfMedians = (arr, left, right, compare) => {
let numMedians = Math.ceil((right - left) / 5);
for (let i = 0; i < numMedians; i++) {
let subLeft = left + i * 5;
let subRight = subLeft + 4;
if (subRight > right) {
subRight = right;
}
let medianIdx = selectIdx(arr, subLeft, subRight, Math.floor((subRight - subLeft) / 2), compare);
let temp = arr[medianIdx];
arr[medianIdx] = arr[left + i];
arr[left + i] = temp;
}
return selectIdx(arr, left, left + numMedians - 1, Math.floor(numMedians / 2), compare);
};

const defaultCompare = (a, b) => {
return a < b ? -1 : a > b ? 1 : 0;
};
``````