BFS
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| class Solution {
private static final int[][] DIRECTIONS = new int[][]{{1, 0}, {-1, 0}, {0, 1}, {0, -1}};
public int[][] updateMatrix(int[][] mat) {
int m = mat.length, n = mat[0].length;
Queue<int[]> queue = new LinkedList<>();
int[][] res = new int[m][n];
for (int i = 0; i < m; i++) {
for (int j = 0; j < n; j++) {
if (mat[i][j] == 0) {
queue.offer(new int[]{i, j});
} else {
res[i][j] = -1;
}
}
}
while (!queue.isEmpty()) {
int[] point = queue.poll();
for (int[] direction : DIRECTIONS) {
int nextI = point[0] + direction[0];
int nextJ = point[1] + direction[1];
if (nextI >= 0 && nextI < m && nextJ >= 0 && nextJ < n && res[nextI][nextJ] == -1) {
res[nextI][nextJ] = res[point[0]][point[1]] + 1;
queue.offer(new int[]{nextI, nextJ});
}
}
}
return res;
}
}
|
以上是多源 BFS。
DP
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| class Solution {
private static final int[][] DIRECTIONS = new int[][]{{1, 0}, {-1, 0}, {0, 1}, {0, -1}};
public int[][] updateMatrix(int[][] mat) {
int m = mat.length, n = mat[0].length;
int[][] dp = new int[m][n];
for (int i = 0; i < m; i++) {
for (int j = 0; j < n; j++) {
if (mat[i][j] == 1) {
dp[i][j] = Integer.MAX_VALUE / 2;
}
}
}
for (int i = 0; i < m; i++) {
for (int j = 0; j < n; j++) {
if (i - 1 >= 0) {
dp[i][j] = Math.min(dp[i][j], dp[i - 1][j] + 1);
}
if (j - 1 >= 0) {
dp[i][j] = Math.min(dp[i][j], dp[i][j - 1] + 1);
}
}
}
for (int i = m - 1; i >= 0; i--) {
for (int j = n - 1; j >= 0; j--) {
if (i + 1 < m) {
dp[i][j] = Math.min(dp[i][j], dp[i + 1][j] + 1);
}
if (j + 1 < n) {
dp[i][j] = Math.min(dp[i][j], dp[i][j + 1] + 1);
}
}
}
return dp;
}
}
|
References
542. 01 Matrix
剑指 Offer II 107. 矩阵中的距离