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前言

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dfs和bfs

一、力扣84. 柱状图中最大的矩形

class Solution {public int largestRectangleArea(int[] heights) {int[] arr = new int[heights.length+2];int res = 0;for(int i = 0; i < heights.length; i ++){arr[i+1] = heights[i];}Deque<Integer> deq = new LinkedList<>();deq.offerLast(0);for(int i = 1; i < arr.length; i ++){if(arr[i] >= arr[deq.peekLast()]){deq.offerLast(i);}else{while(arr[i] < arr[deq.peekLast()]){int right = i;int mid = deq.pollLast();int left = deq.peekLast();int w = right - left -1;int h = arr[mid];res = Math.max(res, w * h);}deq.offerLast(i);}}return res;}
}

二、力扣797. 所有可能的路径

class Solution {List<List<Integer>> res = new ArrayList<>();List<Integer> path = new ArrayList();boolean[] flag;public List<List<Integer>> allPathsSourceTarget(int[][] graph) {flag = new boolean[graph.length];path.add(0);flag[0] = true;dfs(graph,0);return res;}public void dfs(int[][] graph, int cur){if(cur == graph.length-1){res.add(new ArrayList<>(path));return;}for(int i = 0; i < graph[cur].length; i ++){int index = graph[cur][i];if(flag[index]){continue;}flag[index] = true;path.add(index);dfs(graph,index);flag[index] = false;path.remove(path.size()-1);}}
}

三、力扣「代码随想录」DFS，BDF 你没注意的细节都给你列出来了！

class Solution {public int numIslands(char[][] grid) {int res = 0;int m = grid.length, n = grid[0].length;boolean[][] flag = new boolean[m][n];int[][] dir = new int[][]{{0,1},{0,-1},{-1,0},{1,0}};Deque<int[]> deq = new LinkedList<>();for(int i = 0; i < m; i ++){for(int j = 0; j < n; j ++){if(!flag[i][j]){if(grid[i][j] == '1'){deq.offerLast(new int[]{i,j});flag[i][j] = true;res ++;while(!deq.isEmpty()){int[] cur = deq.pollFirst();for(int k = 0; k < 4; k ++){int x = cur[0] + dir[k][0];int y = cur[1] + dir[k][1];if(x < 0 || x >= m || y < 0 || y >= n){continue;}if(grid[x][y] == '1' && !flag[x][y]){deq.offerLast(new int[]{x,y});flag[x][y] = true;}}} }else{flag[i][j] = true;}}}}return res;}
}

四、力扣200. 岛屿数量

class Solution {boolean[][] flag;int[][] arr = new int[][]{{0,1},{0,-1},{-1,0},{1,0}};public int numIslands(char[][] grid) {int res = 0;int m = grid.length, n = grid[0].length;flag = new boolean[m][n];for(int i = 0; i < m; i ++){for(int j = 0; j < n; j ++){if(!flag[i][j] && grid[i][j] == '1'){res ++;flag[i][j] = true;dfs(grid,i,j);}}}return res;}public void dfs(char[][] grid, int x , int y){for(int i = 0; i < 4; i ++){int curX = x + arr[i][0];int curY = y + arr[i][1];if(curX < 0 || curX >= grid.length || curY < 0 || curY >= grid[0].length){continue;}if(!flag[curX][curY] && grid[curX][curY] == '1'){flag[curX][curY] = true;dfs(grid,curX,curY);}}}
}