【数组】二维数组的一些遍历技巧

[miracles1919]

参考：二维数组的花式遍历技巧

[miracles1919]

832. 翻转图像

先来道简单的，水平翻转

/**
 * @param {number[][]} image
 * @return {number[][]}
 */
var flipAndInvertImage = function (image) {
    const n = image.length
    for (let i = 0; i < n; i++) {
        reverse(image[i])
    }
    return image
};

function reverse(arr) {
    let i = 0, j = arr.length - 1
    while (i < j) {
        [arr[i], arr[j]] = [arr[j], arr[i]]
        i++
        j--
    }
    for(let i = 0; i < arr.length; i++) {
        arr[i] ^= 1
    }
}

[miracles1919]

48. 旋转图像

先将矩阵沿对角线翻转，再水平翻转

/**
 * @param {number[][]} matrix
 * @return {void} Do not return anything, modify matrix in-place instead.
 */
var rotate = function (matrix) {
    const n = matrix.length
    // 对角线翻转
    for (let i = 0; i < n; i++) {
        for (let j = i; j < n; j++) {
            let temp = matrix[j][i]
            matrix[j][i] = matrix[i][j]
            matrix[i][j] = temp
        }
    }

    // 水平翻转
    for (let i = 0; i < n; i++) {
        reverse(matrix[i])
    }
    return matrix
};

function reverse(arr) {
    let i = 0, j = arr.length - 1
    while (i < j) {
        let temp = arr[i]
        arr[i] = arr[j]
        arr[j] = temp
        i++
        j--
    }
}

[miracles1919]

54. 螺旋矩阵

按照从左到右、从上到下、从右到左、从下到上的顺序遍历数组，并用四个变量确定未遍历元素的边界

/**
 * @param {number[][]} matrix
 * @return {number[]}
 */
var spiralOrder = function (matrix) {
    const m = matrix.length, n = matrix[0].length
    let top = 0, bottom = m - 1
    let left = 0, right = n - 1
    const res = []
    while (res.length < m * n) {
        if (top <= bottom) {
            // 在顶部从左往右遍历
            for (let i = left; i <= right; i++) {
                res.push(matrix[top][i])
            }
            top++
        }

        if (left <= right) {
            // 在右侧从上往下遍历
            for (let i = top; i <= bottom; i++) {
                res.push(matrix[i][right])
            }
            right--
        }

        if (top <= bottom) {
            // 在底部从右往左遍历
            for (let i = right; i >= left; i--) {
                res.push(matrix[bottom][i])
            }
            bottom--
        }

        if (left <= right) {
            // 在左侧从下往上遍历
            for (let i = bottom; i >= top; i--) {
                res.push(matrix[i][left])
            }
            left++
        }
    }

    return res
};

[miracles1919]

59. 螺旋矩阵 II

/**
 * @param {number} n
 * @return {number[][]}
 */
var generateMatrix = function (n) {
    let top = 0, bottom = n - 1
    let left = 0, right = n - 1
    const res = new Array(n).fill(1).map(_ => [])
    let num = 1
    while (num <= n * n) {
        if (top <= bottom) {
            for (let i = left; i <= right; i++) {
                res[top][i] = num++
            }
            top++
        }
        if (left <= right) {
            for (let i = top; i <= bottom; i++) {
                res[i][right] = num++
            }
            right--
        }
        if (top <= bottom) {
            for (let i = right; i >= left; i--) {
                res[bottom][i] = num++
            }
            bottom--
        }
        if (left <= right) {
            for (let i = bottom; i >= top; i--) {
                res[i][left] = num++
            }
            left++
        }

    }
    return res
};Ï